Improve virtual desktop selection for transients

a) When a group-transient is modal, it still needs to be
   on the current or all virtual desktops if *any* of the
   blocked clients is

BUG: 354090
FIXED-IN: 5.5

b) ignore demanded virtual desktop for transients. Notably modal transients
   should appear where their parent is, and not drag that around. All others
   also better show up above their parent and not a distant virtual desktop

REVIEW: 125758