Add PCN theme system

Coexist with the old theme system for now. QML components have to be
updated to use the new system.

src/config/settings.py

@@ -28,6 +28,7 @@ class General:
     # For Flatpak, it is
     # "~/.var/app/io.github.mirukana.mirage/data/mirage/themes".
     theme: str = "Midnight.qpl"
+    new_theme: str = "test.py"
 
     # Interface scale multiplier, e.g. 0.5 makes everything half-size.
     zoom: float = 1.0