Improve the default project theme

The new default project theme uses StyleBoxFlat extensively for
a more modern design and better scalability to multiple resolutions.

SVG icons are now used in place of PNG icons. While this does not
allow for true vector-based icon drawing (icons are still rasterized
at load-time), this makes the design work easier for contributors
and opens the door to vector drawing in the future (e.g. with polygons
or SDFs).

Like for editor icons, the SVG header file is now built automatically
when a SVG file is changed. This removing the need for running
`make_header.py` manually (TODO).

The "Use Hidpi" project setting has been removed in favor of a
"Default Theme Scale" project setting, which allows creating the
default theme at a higher/lower scale than the default.
This can be used when designing GUIs with a high base resolution
to ensure crisp visuals.

Co-authored-by: Yuri Sizov <yuris@humnom.net>

editor/editor_themes.cpp

@@ -112,6 +112,7 @@ static Ref<Texture2D> flip_icon(Ref<Texture2D> p_texture, bool p_flip_y = false,
 }
 
 #ifdef MODULE_SVG_ENABLED
+// See also `generate_icon()` in `scene/resources/default_theme.cpp`.
 static Ref<ImageTexture> editor_generate_icon(int p_index, bool p_convert_color, float p_scale = EDSCALE, float p_saturation = 1.0, Dictionary p_convert_colors = Dictionary()) {
 	Ref<ImageTexture> icon = memnew(ImageTexture);
 	Ref<Image> img = memnew(Image);

editor/icons/editor_icons_builders.py

@@ -8,7 +8,7 @@ import os
 from io import StringIO
 from platform_methods import subprocess_main
 
-
+# See also `scene/resources/default_theme/default_theme_icons_builders.py`.
 def make_editor_icons_action(target, source, env):
 
     dst = target[0]