Add paired devices on first list

The main window has a list of devices which are paired. If a devices is
disconnected, a dialog will be shown and the device item will be
disabled on the list.
Add devices found on discovery list

Add all devices found (not connected) on discovery.
Add handler to InterfacesAdded/Removed signals

The signal handler was implemented to add/remove devices from device
discovery list.
Add pair to device action

Pair to device when the item from device discovery list is clicked.
Add action to start/stop device discovery

When "search" or "start" buttons are clicked they will start the device
discovery. The stop discovery is done when the "stop" or "back" buttons
are clicked.