Replace theme assets with SVGs

Tracking the ability to use SVGs for theme assets.

Some related efforts:

• #7667
• #7708
• Source artwork for default theme: https://github.com/LMMS/artwork/tree/master/Default%20Theme

Some unresolved issues:

• embed::getIconPixmap() quality is "stuck" at an SVGs "natural" resolution.
  • Fixed via #7769

High level tasks: (break these into finer tasks as needed)

• Switch common assets from .png to .svg
• Decide how to handle text (e.g. "ENVELOPE", "LFO")
• Switch native effects plugins from .png to .svg
• Decide how to handle instrument plugins

Initial team (gathered from Discord)

• @StakeoutPunch
• @rubiefawn
• @Monospace-V
• @SpomJ
• @qnebra (this is not exhaustive, anyone feel free to chime in if you can help!)