Reorganize code related to editor theming

This change introduces a new EditorThemeManager class
to abstract theme generatio and its subroutines.

Logic related to EditorTheme, EditorColorMap, and editor
icons has been extracted into their respective files with
includes cleaned up.

All related files have been moved to a separate folder to
better scope them in the project. This includes relevant
generated files as well.

scene/theme/default_theme.cpp

@@ -78,7 +78,7 @@ static Ref<StyleBoxFlat> sb_expand(Ref<StyleBoxFlat> p_sbox, float p_left, float
 	return p_sbox;
 }
 
-// See also `editor_generate_icon()` in `editor/editor_themes.cpp`.
+// See also `editor_generate_icon()` in `editor/themes/editor_icons.cpp`.
 static Ref<ImageTexture> generate_icon(int p_index) {
 	Ref<Image> img = memnew(Image);
 

scene/theme/default_theme_builders.py

@@ -1,8 +1,10 @@
-"""Functions used to generate source files during build time
+"""
+Functions used to generate source files during build time
 
 All such functions are invoked in a subprocess on Windows to prevent build flakiness.
 
 """
+
 import os
 import os.path
 from platform_methods import subprocess_main

scene/theme/icons/default_theme_icons_builders.py

@@ -1,4 +1,5 @@
-"""Functions used to generate source files during build time
+"""
+Functions used to generate source files during build time
 
 All such functions are invoked in a subprocess on Windows to prevent build flakiness.