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:
* [x] `embed::getIconPixmap()` quality is ["stuck"](https://github.com/user-attachments/assets/dd75605d-4d55-4efc-b6ef-6852c9f3daf4) 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!)