[lib] Remove useless Q_DECLARE_INTERFACE lines for non-plugin classes

These serve no purpose, and break the use of qobject_cast unless
Q_INTERFACES is used, which none of the subclasses do.
[ngf] Fix crash and fragility with handlers in NGF ringtone plugin

The currentCall variable would hold the most recently added handler, and
was cleared when that stopped ringing. In some cases, that could be null
or invalid when another handler's status changed, resulting in a crash.
[ngf] Fix casting -1 to unsigned for invalid event IDs

The event ID is quint32. An error or invalid event is ID 0, not
(unsigned)-1.
[ngf] Stop the ringtone instead of pausing when silenced

Since there is no way it can be resumed, there's no reason to pause
instead of stop the ringtone. Since the event ID was also cleared, the
event would actually never be stopped.
Merge pull request #36 from special/ngf-cleanup

Fix crashes and bugs in NGF ringtone plugin