printers: Make driver installation dialog modal

Pass XID of correct window to InstallPrinterDrivers and
InstallPackageNames methods.

https://bugzilla.gnome.org/show_bug.cgi?id=740115
printers: Use correct type for window id

Window id should be unsigned int and not int. Also the default
value should be 0 instead of the 631 (a copy paste error).

https://bugzilla.gnome.org/show_bug.cgi?id=740115