[applets/appmenu] Improve menu feel
Summary: The appmenu applet now accurately replicates the hover behaviour of a regular menubar.
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
Differential Revision: https://phabricator.kde.org/D29465