[packagekit] invalidating of cache

[Ken VanDine]

I think currently PK invalidates cache after doing an update-system.  Am
I right?  Not sure if anything else does that... but I would like a way
to emit something from the backend to tell it not to invalidate the
cache.  Since we check for updates online, not via the cache it is
rather meaningless to invalidate that.  Something like "invalidate-cache
False".

Thoughts?

--Ken