Add instance variables locate_url and submit_url and setter functions
for them. Set these URLs in the initializers of child classes GClue3G
and GClueWifi.

Do separate connectivity checks for submit_url and locate_url using
g_network_monitor_can_reach_async(). This allows location and submission
servers running on localhost to be reached when there is no outside
connectivity.

Add a GCancellable priv->cancellable and use it in
g_network_monitor_can_reach_async() calls.