AVALANCHE: Implement MainMenu::centre().

Also rework connected functions.
FULLPIPE: Implement sceneHandler18()
FULLPIPE: Implement sceneHandler19()
FULLPIPE: Enable scene18 and scene19
AGOS: Fix error when dumping all VGA scripts.
AGOS: Switch keyboard shortcuts for dumps, to help prevent future conflicts.
VOYEUR: Some renaming related to the computer buttons
VOYEUR: Remove _field26 from the event timer
VOYEUR: Fix logical bug related to the computer data and positioning
FULLPIPE: Implement sceneHandler18and19_showManJump()
FULLPIPE: Implement sceneHandler18and19_boyJumpTo()
FULLPIPE: Implement sceneHandler19_updateNumRides()
AVALANCHE: Implement MainMenu::wait().

Rework other pieces of the engine to fit to it's mechanism.
VOYEUR: Rename some computer fields
AVALANACHE: Rework use of MainMenu.
AVALANCHE: Rework use of ShootEmUp.
TSAGE: Rename savegame header members
TUCKER: Reduce the scope of some variables
TUCKER: Remove some useless modification of sprite state
VOYEUR: Update engine status in header comment