use QDBusServiceWatcher instead of pinging the client

Summary:
the motivation here is to terminate the "server" helper when the client
disappears. dbus supports this use case natively via service registration
events which we can easily handle via QDBusServiceWatcher.
instead of repeatedly poking the client we'll simply monitor its dbus
service now. this is cheaper, less code and doesn't risk timing out
randomly.

Test Plan:

• on neon ISO build kpmcore & calamares & pm
• calamares manages to actually partition stuff
• partitionmanager also starts properly

• also the same again on the installed system.

Reviewers: stikonas, bshah

Reviewed By: bshah

Subscribers: bshah

Differential Revision: https://phabricator.kde.org/D14646