FxMixer GUI updates

- Make the fader persistent peak colours CSS-configurable
- Make the fader background transparent so that it looks good on both inactive and active channels (no discolored bg showing through)
- Make the fxline background CSS-configurable instead of palette-dependent (allows easier styling and cooler effects)
- Increase width of FxLine by 1 pixel so that elements can be centered better
- Add object name to "new channel" button so that it can be styled independently
- Replace the "new" text on the button with a nice and stylish icon, which can also be themed
- Increase size of the send button gfx so that themes can use larger buttons there if they want to
- Some modernization and improvement of the code (eg. use static qpixmaps for graphics in elements that get loaded many times, so we don't waste resources by having duplicate copies of the same bitmap)