Background activity with progress bars

[R4wizard]

Currently when handling a save/load we can lock the main UI thread and the user may think the program has crashed (this is more apparent on slower drives and bigger save files).

We should move all this processing into a background activity and display a progress bar while this is happening.

• [ ] Move save/load code into a background activity
• [ ] Add progress bars to UI when in background activity
• [ ] Disable buttons when in background activity