Fix other themes

[Jowan-Spooner]

• Removes most of the old themes
• Renames "Modern" to "Default Dark"
• Recreates the "Classic" theme
• Adds a simple theme-variant system
  • Creates a "Default Light" theme variant of the Default Dark theme

ToDo:

• follow up Icons PR, where all icons are replaced with a single svg icon sheet.
• Some more cleanup in the themes folder
• Possibly implement theme-scaling as an alternative to content_scale scaling.

[Calinou]

Possibly implement theme-scaling as an alternative to content_scale scaling.

Out of curiosity, why is that needed? godotengine/godot#86022 should make a custom approach unnecessary once it's merged, as it'll be possible to easily use hiDPI-friendly icons out of the box.

[Jowan-Spooner]

@Calinou Yeah, I know, I would love to use that. But it's been stale for another half year. I'll try to keep my workarounds easily undoable because of course the built-in solution would be best, but it's just not there yet. So I will create workarounds until then. Rod has considered removing the multi-window support because of how buggy scaling is with sub-windows and I think that would be a real bummer, so I've been experimenting with alternative ways to get better scaling. Anyways the theme-scaling isn't in this PR anyways, it's just something that could be possible with this approach.

Also this has other benefits, such as recoloring icons, which isn't possible otherwise.