Document Copy/MoveSaveSlot(), new DeleteSaveSlot() behavior

[ericoporto]

https://github.com/adventuregamestudio/ags-manual/wiki/Globalfunctions_General#deletesaveslot

• Added CopySaveSlot() and MoveSaveSlot(), which moves existing save to another slot.
• Document that DeleteSaveSlot() no longer rearranges the slots (from PR Script API: expand methods for displaying list of files and saves ags#2541)
  • Calling DeleteSaveSlot() on a slot within 0-50 range will no longer secretly move a save with the topmost number (within the same range) to fill the emptied slot.

/// Copies the save game from one slot to another, overwriting a save if one was already present at a new slot.
import void CopySaveSlot(int old_slot, int new_slot);
/// Moves the save game from one slot to another, overwriting a save if one was already present at a new slot.
import void MoveSaveSlot(int old_slot, int new_slot);

[ericoporto]

Still need to document the behavior change in DeleteSaveSlot, but added new functions here
https://github.com/adventuregamestudio/ags-manual/wiki/_compare/a36fe7a5730138438493a010d2f8e25b906f8a8e...fef7be5aa697621462a0499966353329d9363eb4

Probably need to also think of an example that reproduces the old behavior using movesaveslot