Fix custom Text Editor theme changes on reload

[kitbdev]

• Fixes Modifying colors of the text editor theme highlightings causes non-modified colors to change without user input upon project reload. #103914

When the EditorSettings loads defaults, I replaced the call to _load_godot2_text_editor_theme with EDITOR_SETTING*, this is so the text editor theme settings always saves to the editor settings file, just like the editor theme settings does. This is important when loading and the preset is Custom.
The _load_godot2_text_editor_theme should probably be moved to the editor theme manager in the future, for now I left it as is since the text editor theme loading still uses it.

I wanted to have the EditorSettingsDialog immediately update so the revert icons show, but I ran into an issue with the ColorPickerButton completely freezing the dialog if it was open while reloading. As before, they only reload when reopening the category or the dialog. It shouldn't cause any issues that they aren't immediately updated, since it is just for the revert buttons.

The initial values for the Custom preset are now the current Default theme. So if you were on Godot 2 and change to Custom, all of the values can be reverted, and the same from Default if the editor's base color changes, since Default changes based on it.

[KoBeWi]

Why do _load_defaults() and _load_godot2_text_editor_theme() initialize the same values?

[kitbdev]

I plan to move _load_godot2_text_editor_theme to EditorThemeManager or at least change it to use set_initial_value instead. _load_godot2_text_editor_theme is used when changing the theme, but the first initializing needs to use EDITOR_SETTING.
The color for the base theme does similar, there is a value set here in EditorSettings, and again in EditorThemeManager.

[KoBeWi]

You can introduce some struct Godot2Theme or something, which would have constants to use for default values. Defining them twice is not very maintainable.

Although if you plan to clean it up then it's probably fine for now. This fixes the bug at least.

[Repiteo]

Thanks!