Add display selection to graphics options

[frosch123]

This is the same as #6023 . The bots couldn't handle the rebase/force-push there.

Type of change
New feature

New behavior

• New setting "display" in the options, next to the "window size".
• On systems with multiple displays, this specifies:
  • The display to show the main window on, when widelands starts. ("preferred display")
  • The display to use, if fullscreen is enabled.
• Possible values for the setting are "follow mouse" or selecting a specific display.
  • The default "Follow mouse" means: when starting widelands, the main window will be shown on the display, the mouse pointer is on.

Screenshots