Port Appmenu model to QDBusServiceWatcher

Summary:
QDBusConnection::serviceOwnerChanged is deprecated, with the intended
replacement being QDBusServiceWatcher. QDBusServiceWatcher only creates
a match rule for the relevant services, resulting in less noise and
wakeups.

(practically this has no impact as the the system tray also connects to
watch every service change, and that runs in the same process)

Test Plan:
Ran appmenu
Had dolphin open showing the menu
Closed dolphin
menu immediately closed

Reviewers: #plasma, broulik

Reviewed By: #plasma, broulik

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D7611