Apply MSDF fonts theme to AnimationTreeEditor

[StamLord]

This PR is an attempt to address #91814

As suggested by @Calinou, I based my solution on the approach used in visual_shader_editor_plugin.cpp.

Changes included:

• Added a local Ref object I instantiate in AnimationTreeEditor, instantiated in the constructor
• Populated the theme in a new update_theme() function
• Called update_theme() from the constructor, immediately after creating the theme, since AnimationTreeEditor does not have an _update_graph function like in VisualShaderEditor
• Applied the theme to AnimationTreeEditor itself (set_theme) so that all child AnimationTreeEditorPlugin inherit it.
  Applying it to the individual plugins also works, but seems redundant.

This replaces the default font with and MSDF font that scale better.

[Calinou]

Tested locally, it works as expected.

Some feedback:

• The font should only applied to GraphEdit, not the rest of the AnimationTree editor. This prevents fonts such as this one from looking different (and blurrier) than the rest of the editor:

In other words, MSDF fonts should only be used where the zoom level can dynamically be changed.

[StamLord]

Thanks @Calinou I moved the fix to GraphEdit and it seems to work:

Can you shed some light on how GraphEdit relates to the AnimationTreeEditor? Is it the parent or is there some inheritance going on that I didn't find?

[StamLord]

I've been wrestling with the pre commit hooks a bit.

• I wrapped the headers, GraphEdit::update_theme and the set_theme call in #ifdef TOOLS_ENABLED so it will not fail non-editor builds
  *I added a null check of the editor singleton inside set_theme since it failed on headless Linux builds (maybe due to singleton not being ready yet?)

I'd appreciate someone taking a closer look at my workarounds since maybe there's a simple way to handle this.

Question: since GraphEdit is a usuable node in a shipped game, should the default font be msdf outside the editor as well?
MSDF will look better by default but we might want to leave theme control up to the user.

[Calinou]

Files in scene/ should not have dependencies on editor code, so I'm afraid this isn't the correct way to fix the issue. The previous approach was more logical from a technical perspective; it just needed to be more scoped to avoid affecting the entire AnimationTree editor (and affect just the GraphEdit that's within).

Question: since GraphEdit is a usuable node in a shipped game

GraphEdit/GraphNode are also usable in exported projects, e.g. in Material Maker.