[packagekit] Adding support for "prepare" in transaction_flags

[Richard Hughes]

Hi all,

The new transaction_flags has a new bit that might be set. If the
PREPARE bit is set, then it's important that the transaction that the
backend has been asked to do is setup, all the files downloaded, but
changes actually not made. This is different to simulate, as simulate
doesn't actually download packages.

We're intending to use this heavily in GNOME 3.6, so it's kinda
important to implement in the next few months. I've done zif, and I'll
do yum on Monday with Nils.

Any questions, yell. Thanks.

Richard.