Lutris: Add screen resolution variable to the installers

[tannisroot]

So that instead of enabling Virtual desktop (which is absolutely needed for a lot of Unity games) like this:

winesteam:
  Desktop: true
  WineDesktop: 1920x1080

I could just do this:

winesteam:
  Desktop: true
  WineDesktop: $RESOLUTION

where in place of $RESOLUTION actual resolution of the machine's main display would be parsed.

[Medath]

I think an auto option would be better than using a variable, which would write the actual resolution into the config, because then lutris would be able to dynamically choose the resolution depending on the monitor that's currently used (So the user wouldn't have to change the config when getting a new monitor or something)

[strycore]

@Medath this is not possible. What @tannisroot suggests is correct.

[strycore]

@tannisroot I added the variables but I also added a note in the docs about not using them for virtual desktops.