Update Fullscreen handling.

[dscalzi]

The fullscreen argument no longer works 100% properly on at least 1.15+ (probably a 1.13+ issue, need to test with that version).

A fix should be manually saving fullscreen: true to the options.txt when the setting is ticked on. This should be done as a prelaunch task to make sure the value is properly set. This would have to be done as a pre launch task. The only issue is that this would be applied to all servers, and it wouldnt be possible to have a per server configuration for this value.

Another option is removing the fullscreen value from the launcher settings and delegating it to the in game settings only.