application: Add 'connected' property

Add connected property to application.js and check network to set
property true/false.

https://bugzilla.gnome.org/show_bug.cgi?id=737221
mainWindow: no-network-view on no connection

Check for connected property and switch to a 'no-network'
view when not connected.

https://bugzilla.gnome.org/show_bug.cgi?id=737221