[voicecall] Don't load plugins if configuration fails.

Use this feature to pass on DBus service registration failure.
[voicecall] Remove QtSingleApplication usage.

There is absolutely no need to use this just for keeping a single instance
running. We can already do that by looking at whether the DBus registration
succeeds or not.
[voicecall] Always automatically restart manager and UI.
Merge pull request #5 from rburchell/master

General fixes and removal of QtSingleApplication