[New Feature] Auto-save in singleplayer

[Rampastring]

Description:

Related to #25 . ts-patches can also auto-save singleplayer missions with a user-configurable delay between auto saves and a specific number of auto-save slots (currently 3 in ts-patches). Once all slots are filled, new auto-saves will overwrite saves from previous slots, starting from the oldest save.

Possible Implementation:

Unlike with the multiplayer implementation, we don't need to perform the actual SAVEGAME event in singleplayer. Simply saving directly will do.

Additional Files:

CnCNet ts-patches implementation: https://github.com/CnCNet/ts-patches/blob/master/src/Hook_Main_Loop.c#L79

The CnCNet spawner also reads the values of AutoSaveGame and NextSPAutoSaveId from spawn.ini, allowing the client to set the delay between auto-saves and the initial auto-save ID. Setting of the initial ID is necessary for the logic to function properly with saved games. For example, if the user loads the save in auto-save slot 2, the next auto-save should be written to slot 3, not to slot 1.

DTA show-case:
https://www.moddb.com/mods/the-dawn-of-the-tiberium-age/images/single-player-auto-save#imagebox