* Basic logic for layer effects
* Add an FX button and the ability to add effects, no way to remove or change properties of effects yet
* Basic and ugly UI for adding and removing effects, no property changing yet
* Swap effects
* Fix preload shader paths
* Change parameters for layer effects
* Change gradient parameter in layer effect shaders, and other fixes
* Use CollapsibleContainers for the shader properties
* Set the correct gradient interpolation mode and color space in the UI
* Make effects of group layers apply to children
* Change `apply_fx` to `apply_effects`, formatting, some extra doc comments
* Apply effects to other canvases, when merging layers and when exporting
* Display humanized names of the shader unifrms
* Some UI improvements to the LayerEffectsSettings
* Add an Enabled button in the layer effects window, and change checkboxes to checkbuttons
* Change BaseLayer.apply_effects() to take a cel as a parameter instead
* Make layer effect buttons be affected by the modulate icon color
* Add option in the View menu whether layer effects are displayed in the canvas or not
* Rename `apply_effects()` to `display_effects()`
* Add translation strings
* Add nearest filter to the gradient map
* Don't change Main.tscn
* Fix more translations
* Change the default cursor shape of the generated UI elements of the layer effects
* Add undo/redo and effect application (apply effect destructively)
There are some errors due to the usage of anonymous lambda methods in undo/redo, but it seems to be working well regardless.
* Make layer effect application work on all cels
