Release v0.9.99-17

Multi-level support (experimental)

• It's possible to load more than one level at once.
  • A new window is added to manage loaded levels.
  • A new option is added to load new levels as tabs or replace existing ones.
  • Two new shortcuts are added to navigate between loaded levels (not bound by default).

Other changes

• Removed the quick saving screen and is replaced with a throbber text in the lower left to the screen, to be less desruptive.