Improve editor theme generation after the refactor

[YuriSizov]

Follow-up to #87085.

• Fixes a bug with the color mode of icons and fonts not respecting the color preset.
• Fixes a bug with the benchmark by moving benchmarking code to the main class and tracking the number of runs.
• As a cleanup step, moves is_dark_theme() from EditorSettings to EditorThemeManager.
• Adds a hashing mechanism to easily compare editor settings affecting the theme generation.
• Updates the icon generation to use this mechanism, and adds notes for future improvements to fonts and styles.
• Properly tracks the subresource_hue_tint setting and accounts for it when deciding to regenerate the theme.
• Removes EDITOR_GETs from editor_icons.cpp to better consolidate them in the main file.
  • This also needs to be done in some way to editor_fonts.cpp, but that's a more complex task for now.
• Some less notable cleanup.

[akien-mga]

Looks good to me from a cursory review.

[akien-mga]

Thanks!