New feature: UI customization

[aapis]

This one's a bit of a doozy, but it allows for nearly full customization of the UI.

• Adds new parent above the normal sidebar to allow Settings to move out of the window it currently occupies into the UI
• Adds appearance tab. Supports choosing a "window style", uploading images from ~/Pictures for background images, solid colour backgrounds, customizing the accent colour and choosing the colour mode.
• Most higher level UI elements have been modified to support the 4 "window style" (classic, opaque, hybrid and glass. Each represents an increasing amount of transparency)
• Created 4 wallpapers using https://github.com/aapis/genevive to show how different colours interact with the UI

Customization can make the UI very difficult to read, but if that's what you want to do to yourself I won't stop you.