network: When processing new devices, look at the active connection first

NMConnection._sync() expects the active connection to already be in _connectionItems, which happens from _addConnection(). If it isn't in there, we get a null dereference and the indicator can get into a weird state where it doesn't display devices / connections properly.

When processing a new device in NMApplet._deviceAdded(), let's look for an active connection and add that one first before looking at the rest of the connections.

Closes: #140 (closed)