Use GCancellable when setting a new value of an option.
This prevents Printers panel from crash caused by
calling of option widget's callback on finalized widget.
https://bugzilla.gnome.org/show_bug.cgi?id=694874
Add Options dialog which allows users to set more options than current dialog.
The dialog reads printer's PPD file and add its options to the dialog together
with some preselected IPP options (#678637).
All operations in the dialog are asynchronous.
During implementation of this, the option for setting allowed users was removed
because this is not suitable for this panel (the option is intended for
administrators).
