Begin the Avalonia migration

[jonko0493]

Yeah, it's a big one.

This deletes the entire (!) UI project. In it's place, we're building an Avalonia application. So far, the following things have been implemented:

• Home panel
• About dialog
• Preferences dialog
• Update available dialog
• New project dialog, project creation
• Open project panel (item explorer & editor tabs)
• Background editor
• Image resize/crop dialog (kinda, not working fully)

This will all need a pass from @WiIIiam278 to make it look nice (and work right in some cases).