network: Update the icon in the panel whenever NM's state changes

Similar to what it's done when the main connection changes, we need to make sure that the icon in the panel gets updated before calling _syncConnectivity(), so that the icon gets always updated if needed, regardless of whether there's an active connetion or not.

This is needed because there's at least one case when an icon should be shown with the computer is not connected to any network: when a hotspot has been enabled, which can be useful even if there's not an internet connection to share (e.g. to easily allow connecting other devices to the computer.

Closes: #214 (closed)