Map reloads counter - new processor variable

[cardillan]

Adds a @mapReloads processor variable. The variable is set to zero on code compilation, and gets incremented each time a map is loaded. It serves for easy detection of map reload events (either from a save file, or by switching a sector in campaign). Since display content is not stored in the map file, this allows programs that update their display incrementally to detect the display needs to be fully redrawn. There might be other use cases; maybe world processors could trigger some events in custom maps when a map gets loaded.

Reload detection is possible in two ways: firstly, the last value of @mapReloads can be stored in a variable; when a difference is found, the stored value is updated and the display is redrawn. Secondly, since @mapReloads is writable, it is possible to set it back to zero when a non-zero value, and thus a map reload, is detected.

The variable is displayed in the Vars screen, which makes it perhaps more prominent than it deserves. If this is a problem, I'll try to add a fix to hide it from this list. (I've also added it to the list of built-in variables, even though @counter isn't there.)

Notes:

• I'm not aware of any other mechanism to detect map reloads (except "ghost cells", which is not really all that usable), despite asking around. If there already is another way, then this can be scrapped - I'd just like to know what is the way.
• Map reloads could be tracked within the map itself. Adding a just processor variable seemed simpler to me - the code added is minimal. The possibility to reset the flag independently by each processor is a bonus which makes using the variable easier.

If your pull request is not translation or serverlist-related, read the list of requirements below and check each box:

• I have read the contribution guidelines.
• I have ensured that my code compiles, if applicable.
• I have ensured that any new features in this PR function correctly in-game, if applicable.

[1ue999]

seems cool

[cardillan]

This is how I expect the map reload detection to look like:

jump no_reload equals @mapReloads 0
set @mapReloads 0
# the map reload handling code
# redraw the display, etc.
no_reload:

Even if a map gets saved/reloaded again just after executing the jump, with @counter pointing at the set @mapReloads 0 instruction, we won't miss a reload, since at this point the display redraw code gets executed anyway.