Inform the user when no drives are present in BraseroDriveSelection
Don't enable Eject button in eject dialog if no drives available

If no drives are present on the system disable the drive selection combobox
and eject button in the eject dialogue.
Make the Split Track dialogue modal

This prevents the underlying window receiving input meant for the dialogue
Use GtkApplication rather than GApplication

We need a GtkApplication to provide an application menu