applets/systray: align applet labels with differing line counts in hidden view

Discover current minimum height of applet labels in hidden
view and apply it for all of them when they are shown
inside hidden applets view. This implementation provides
always only the maximum needed label height based on label
contents.

BUG: 438347
FIXED-IN: 5.24.3


(cherry picked from commit 8fcdfc30)