Improve account connection handling
- libnotify has been introduced, to notify the user about connection losses and reconnections
- the 'account disconnected' notification shows a button for opening the 'account settings' view
- the spinner of the main view header has been removed. Instead, a spinner has been added to each entry in the accounts-list.
- the handling of the SMS account reconnection is handled by a "modem_added" signal from ModemManager
- the reconnection of accounts has been tested by restarting a jabber server (Prosody), as well as the network connection
closes #188 (closed) and #193 (closed)
@guido.gunther maybe you wanna give it a try