Add an extra button "Clean Print Heads" that will be shown when the relevant
maintenance command is available for a printer, to bring this feature back.
Additionally, this commit adds a 'parameters' attribute to PpMaintenanceCommand,
which is needed as the "Clean" command requires to at least be passed "all" as
its single parameter: https://www.cups.org/doc/spec-command.html#Cleanhttps://bugzilla.gnome.org/show_bug.cgi?id=764620
This cleans the code up a bit so that we can extract part of the logic
from _pp_maintenance_command_execute_thread() before getting into fixing
the problem in the logic checking whether a CUPS command is available.
Besides, it will be useful to have this logic extracted as it will be used
later on from pp-printer-entry.c to know whether the "Clean" command is
available, in order to show a menu item "Clean Print Heads" (bug 764620).
https://bugzilla.gnome.org/show_bug.cgi?id=766861
