ZVISION: Implement several advanced engine features and ScummVM dialogs

The functionality to return to launcher, list saves, delete saves, load
games from the launcher and load and save games during runtime has been
implemented. Also, ScummVM save/load dialogs have been implemented.
Saved games now have three numbers in their file extension, bumping the
possible save game slots up to 999
ZVISION: Add some spacing
ZVISION: Remove unused code