The recent move to the new Shell layout with a sidebar forces
us to relayout all the panels that also use a sidebar. One of
these panels is the Details panel.
The first step to move the Details panel away from the sidebar
is moving the Default Applications page to a new, dedicate panel.
This patch introduces the new Default Applications panel.
printers: Check all supported CUPS commands, not just the first one
Use an array of strings to store every supported command and check
the desired command against the elements in that list, instead of
simply checking the first one.
printers: Added new async API to check availability of maintenance commands
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).
