Fix memory leak in Appmenu when using compact menu

Summary:
::createMenu sometimes creates a new menu, sometimes returns a pointer
to a menu from libdbusmenu that we shouldn't delete.

This patch makes any newly created menu delete itself on closure.

It then shuffles some code so that we only call createMenu after
doing checks.

Test Plan:
Ran in compact mode, clicked menu a few times
Ran in full mode, clicked a menu a few times

Reviewers: Plasma, broulik

Reviewed By: Plasma, broulik

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D4393