Pass a GCancellable object to g_dbus_proxy_call() for DevicesGet
method. This assures that g-c-c won't crash when user cancels addition
of new printer before the call is finished (rhbz#691931).
This patch makes PpNewPrinterDialog to call DevicesGet method
asynchronously now.
It also adds spinner to the lower left corner with label showing
status of this operation (#644212).
The dialog is able to add printers discovered by cupsGetDevices().
It can also add printers from a remote CUPS server by typing
its address and selecting printer we want to add (#640734).
