Refactor & fix bugs.

[knightofiam]

Player:

• Fix some player state machine bugs preventing player from transitioning to
  the correct state due to having multiple valid triggers, which in turn is
  caused by not being specific enough when specifying trigger conditions
  during state machine initialization. This reduces strange bugs where the
  player gets stuck in a specific state or won't take a valid action like
  reading a sign, or the player doesn't respond to a specific valid input.

• Fix tile dropdown bug that was occurring, for example, when standing
  on top of arrow signs, MoveAndSlide is detecting a collision between
  the player and sign, but the player's Area2D animation collider rect
  is not actually overlapping the sign tile, and the reported
  collision point when converted to a tile cell coordinate confirms
  this. Do not drop down when there is no actual tile being
  intersected, even if a collision is being reported. Perhaps in the
  future there could be a one tile cell margin added to detect overlap
  to account for the player being very close to the tile..

• Remove player idle back animation delay when finished reading a sign, at
  least for now. It looked a bit awkward when finished reading a sign from too
  far off center.

Editor / Scenes:

• Simplify main scene by combining upper & lower cliffs to reduce level
  complexity and duplication by almost 50%. Merge lower cliffs into upper
  cliffs and rename upper cliffs to cliffs.

• Restructure the ground colliders to make them more reliable & way less
  buggy.

• Reorganize & rename many nodes in Godot editor for simplicity & readability,
  which was made possible by the merging of upper and lower cliffs.

• Reorganize the 10 waterfall sections so that section 1 is the very top and
  section 10 is the very bottom, respectively, which is a much more intuitive
  layout.

• Lock some nodes in the editor to prevent them from being accidentally
  modified.

Tiles:

• Heavily rework tile intersection code in Tools.cs to improve accuracy and
  eliminate bugs. Allow units per tile aka cell size to vary from 1 to 16 for
  precise placement of tiles without affecting collision and overlap
  detection.

• Greatly simplify player updates in Cliff.cs for detecting both cliff
  enclosing & ice intersection code.

Signals:

• Move ground detection signal callbacks for the player from Cliffs.cs to
  Player.cs which greatly simplifies everything, including not having to add
  duplicate callbacks in both classes.

• Block more signals while changing player's animation-based collision shape
  to prevent signal callbacks from being triggered more than they should be.
  This is an ongoing workaround for
  Changing node parent produces Area2D/3D signal duplicates godotengine/godot#14578 "Changing node parent
  produces Area2D/3D signal duplicates".

• Don't block sign signals anymore because it causes problems with reading the
  sign because it doesn't consistently detect the player is in the sign, and
  it's working fine without the blocking.

• Improve naming of some signal callback methods.

Player.cs refactoring:

• Remove the use of delta from Player.cs, where it mainly a hack; awaiting a
  certain amount of idle frames has been more efficient.

• Add more private convenience methods for energy meter management to simplify
  the code & increase readability.

• Add more private convenience methods for sign reading management to simplify
  the code & increase readability.

• Make fields whose value changes in async methods volatile to prevent the
  wrong value from being read, causing hard to track down bugs.

General refactoring:

• Move more GetNode calls into _Ready methods and assign them to fields for
  efficiency, code simplicity, readability, & less possibilities for bugs.

Tools:

• Don't throw an exception in Tools when checking if any arrow key is pressed
  except the specified one, when the specified input is invalid; just return
  false to decrease game crashes.

Extensions:

• Add a string extension method for allowing the use of the null coalescing
  operator to work with empty strings, which greatly simplifies
  string-handling code, e.g., when using the ternary operator to return
  strings from methods.

Cleanup:

• Remove unused & obsolete classes.
• Remove unused & obsolete methods.
• Remove outdated & unnecessary comments.
• Suppress unnecessary compiler warnings with comments for increased readability.
• Remove obsolete compiler warning suppression comments.
• Rename some outdated variables for increased clarity on their purpose.
• Rename some methods for simplicity.
• Reorganize order of some fields for increased readability>
• Remove obsolete formatter-disabling tags for code blocks.
• Add formatter-disabling tags where necessary.
• Add some TODO's.
• Improve many log messages.
• Improve code formatting.