Do not crash when running the GMainLoop in PkTransactionExtra

There may be some deeper lifecycle issues of the parent object, but seeing as
this has been replaced by a proper plugin runner in newer versions it's not
worth thinking too hard about it. This fix has been tested and found to work.
Do not scan desktop files by default

This was a significant source of crashes in RHEL, and it's only optionally used
in older versions of gnome-packagekit when showing applications-as-packages.
Do not generate the package cache by default

This was unreliable, and only really used in the search-as-you-type
functionality in gnome-packagekit.
Do not scan the session processes by default

We can't do this reliably when sessions are coming and going, and we often show
false-positives.