Some improvements to ApplicationUpdates:
- Assign m_aptBackend in ApplicationUpdates::setBackend(), before it just always remained null.
- Only listen for QApt signals when the update starts. Due to design deficiencies in LibQApt it can be dangerous to listen for signals when we aren't expecting them.
- On that note, don't handle worker events or errors at all. The ApplicationBackend already handles errors/events, so we'll just be stepping over ApplicationBackend when it tries to handle them.

A combination of the last two items was causing a crash in muon-installer when commits finished, even though muon-installer wasn't directly using ApplicationUpdates. :P