Folder View: Make popup dialog wide enough for one more grid cell

Right now it is 2 pixels too narrow, so there is always a huge gap on
the right side. Use a standard Unit that knows how to adjust with the
font size to avoid this, in lieu of being able to get the scrollbar
width.

BUG: 417539
FIXED-IN: 5.24.5
(cherry picked from commit 2ebde103)
2 jobs for Plasma/5.24 in 3 minutes and 6 seconds (queued for 2 seconds)