[ok] new ticky box on scheduling page - only aircraft in maintenance

[sanabas]

As per title. If you have spare planes and are moving schedules from anything in C-check to the spare plane, there's a lot of clicking to find stuff in maintenance. Filter box, same as 'show unscheduled aircraft', would bring them all up quickly in order to click the move schedule button on each.

[Sami]

To-do: Add filter