printers: Update GtkListBox of printers consecutively

Goal of this commit is to not overwhelm UI when a lot of printers is added to CUPS. You can reproduce this situation when you add e.g. 30 printers using lpadmin. Th UI stops to respond for some time.

To do so, the printer entries are not deleted every time there is a request for updating of the list of printers but only new printer entries are added and printer entries of deleted printers are removed. Other printer entries are just updated by new method pp_printer_entry_update(). Which, by the way, is almost whole taken from pp_printer_entry_new().

This needed to add sort function for the list of printer entries. It sorts printers according to their names not taking case into account.

In the similar manner, the filter function was extended to not show printers which are being deleted. This needs a list of names of deleted printers which we keep until they are really deleted. One important thing here is the "reference" object which points to the panel itself via its "self" key. We pass this object to the pp_printer_delete_async()'s callback so it knows whether it can remove the printer's name from the list of deleted printers (once the panel is being destroyed it clears the key itself).

This was originally reported to me in https://bugzilla.redhat.com/show_bug.cgi?id=1700002.