UI Redesign

[ucupumar]

There are a lot of things that ucupaint UI can do better.

• [x] Merge override with layer channel input
• [x] Layer vector options should be visible from the start
• [x] Dropdowns should be easier to click
• [x] Cleaner layer channel dropdown
• [ ] Cleaner layer dropdown
• [ ] Cleaner mask dropdown
• [ ] Cleaner main channel dropdown
• [ ] Show Modifier up, down, and delete from the start
• [ ] Experiment with tab system
• [ ] Update show channel toggles with more representative icon (https://x.com/don570don570/status/1848512087833694469)