Added level loading screen (reference issue #190)

[catprisbrey]

#190

Created a MasterLevelLoadingMenu class and scene, with added logic to the Master MainMenu. It borrows from the Continue game logic as well as the SceneLister logic.

Levels are propagated into an ItemList, with their file names cleaned. Whatever level is activated, will update the GameStates.current_level to match the level selected. A signal emits which triggers the load action from the main menu.

It tried to keep it minimal, and imitate the style of logic already established.

[catprisbrey]

This morning testing more.

I referenced GameState to set the current level, but with testing in other example projects, that reference becomes unavailable, but GameStateExample works.

[Maaack]

Thanks for opening the pull request. I'll have time to test and review in a few days. Short of a critical bug, I'll merge this into main so your contribution is recorded. I'm not sure it'll show up on the front page without GitHub's verification (green check mark next to commits) though. Not sure if that matters to you, but just a heads up.

The GameStateExample is a recent addition and breaks from the project's conventions by adding a class that is purely an example (ie. meant to be re-written rather than inherited). When the example/ folder get's copied, the Example extension is removed and it becomes GameState.

[catprisbrey]

The various locations of examples scenes and the nuance of some of it I still don't think I grasp. For example, i'm uncertain why when i used GameState it stopped working on my other project, while GameStateExample did. There is just enough going on under the hood on your project that I expect you'll tryout my PR, look it over, and find several things needing to be adjusted.

In my continued testing I also found it handy to grab focus to the ItemList, but that isn't part of this PR. I can drop and resubmit and updated PR if you like, or just hold off until you've had a chance to try this version first. I don't collaborate with other's projects on GitHub enough to know what's best for you.

[Maaack]

Totally understand. The copying process is a little unconventional. GameStateExample should be used inside of addons/.... The expectation is that most people use the plugin version from the Godot Asset Library, and then copy the examples into their project's root folder. The copy process can be tested under Project > Tools.

To grab focus, there is a capture_focus.gd script that can be applied to a Control node, and will grab focus when the first selectable UI element in the tree becomes visible. Feel free to add it to your level select scene.

[catprisbrey]

Made the changes, so the scene now uses the capture_focus container node. The LevelSelect screen now uses the GameStateExample to set the new current level before loading it instead of GameState.
I'm going to test these changes more on my own project and ensure there aren't any major issues with it, but so far so good it seems.