Add InputState class and refactor default input implementation

[Xrayez]

Resolves godotengine/godot-proposals#104.
Resolves godotengine/godot-proposals#100.

This is a concrete implementation of the new InputState class which aims to decouple and expose the internal Input state. The workflow remains the same for all Godot devs, so this just makes the Input API more flexible.

It would be good to be able to override InputDefault implementation via modules instead, but I haven't found a way of doing so yet, so consider this proposal to be a demonstration over the limitations I'm dealing with input handling in Godot.

Test project

https://github.com/Xrayez/godot-input-state-demo

[groud]

See my comment there: godotengine/godot-proposals#104 (comment)

[Xrayez]

#37317:

• Input was renamed to InputFilter.
• InputDefault is removed.

These introduce too many changes for this PR to be rebased and it's easier to just redo this, so closing. But also see godotengine/godot-proposals#104 (comment).

You can still test this PR if you wish so, I'm not going to delete the branch for now.