CompactApplet: Only draw separator line for panel applets

The purpose of this line is to separate a panel applet's popup from the
panel, but when there is no panel, the line gets drawn anyway, has
undefined geometry, ends up too big, and looks weird. This commit hides
it for the popups of non-panel applets.

BUG: 439374
FIXED-IN: 5.23


(cherry picked from commit 864b04de)