folder: use qset instead of qvector to improve performance

we need to check N times if the N items associated with a given screen
have already been marked as off-screen. this requires traversal of all
off-screen items. qset gives us much faster performance for this use
case.

the contains() calls in removeScreen change from multiple milliseconds
to sub-millisecond speeds for some 700k items

(cherry picked from commit 2793b1b0)