offline updates: Cancel the update when the user presses Cancel

Due to an early return code path, the offline update would still remain
in the triggered state even after pressing cancel in the shutdown
dialog. Fix this.
offline updates: Fix a race with shutdown dialog showing

Wait for pk-trigger-offline-update to finish before showing the shutdown
dialog; otherwise gnome-shell won't know that it needs to show the
special update installing dialog and just brings up the regular one.
trivial: Drop an unrecognized configure option from distcheck
trivial: Remove an extra newline in a debug message
Call gs_plugin_updates_changed() when another tool touches the package database
trivial: Remove all applications from the cache when updates have changed
Set applications to unknown state when the updates cache is changed
Invalidate the source IDs when installing or removing
Reload pages in the software center when the updates cache is changed

This also makes the different actions more explicit:

_setup:	        Sets up any state for the shell panel; shows no UI
_load:          Does any action and displays the result
_reload:        Invalidates caches, then does _load()
_switch_to:     Sets up headerbar and then does _load()

Resolves: https://bugzilla.gnome.org/show_bug.cgi?id=734437