[voicecall] Fix crash when active voice call ends.

Fix a null pointer dereference which occurs when the
onVoiceCallRemoved() slots is called multiple times in quick
succession, from different providers.
Merge pull request #20 from rburchell/master

Remove dead code
Merge pull request #19 from amccarthy/null-dereference-fix

[voicecall] Fix crash when active voice call ends.
[voicecall] Various fixes and improvements.
Merge pull request #21 from martinjones/master

[voicecall] Various fixes and improvements.
[voicecall] Provide a valid startAt time.
Merge pull request #22 from martinjones/master

[voicecall] Provide a valid startAt time.
Fix warnings and duplication in voicecall code
Merge pull request #23 from jpetrell/master

Fix warnings and duplication in voicecall code
Replace StreamChannelListener in commhistory-daemon by logging events in voicecall.

This involves a lot less fiddling to get properties out of a telepathy stream,
and generally Just Works.

The one part of missing functionality is that there is no periodic commit of
event modifications every 5 minutes, but if desired, that is easy to restore.