[applets/appmenu] Improve menu feel

Summary: The appmenu applet now accurately replicates the hover behaviour of a regular menubar.

Test Plan:
Before: you need to click to engage all menus.
After: the appmenu applet copies the behaviour of a QMenuBar when it comes to clicks and hovers.

Reviewers: Plasma, broulik

Reviewed By: Plasma, broulik

Subscribers: broulik, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D29465