Change: base autosaves intervals on real time (instead of game time) (#10655)

There are two fundamental issues with autosave:
- When fast-forwarding, it saves way too often
- When paused, it never saves

Both makes no sense. Autosaves are meant to prevent you from
accidentally losing your work. The emphasis on "your" work.

To solve both issues, the autosave now works on real time. You
can select every 10 / 30 / 60 / 120 minutes, which are similar to
what the setting was in game-months.

When you pause, autosaving will stop. Unless you make any change
to the game; then it will continue to make autosaves, even so
the game is paused. Unpausing / pausing resets this mechanism.

src/timer/timer_game_calendar.h

@@ -42,7 +42,6 @@ public:
 
 		/* All other may have a Random() call in them, so order is important.
 		 * For safety, you can only setup a single timer on a single priority. */
-		AUTOSAVE,
 		COMPANY,
 		DISASTER,
 		ENGINE,