[packagekit] Adding support for "prepare" in transaction_flags

[Daniel Nicoletti]

> Yes, but the flip side is you have to have an CommitPackages()-style
> mega-method, and you loose the ability to have different argument
> lists for UpdatePackages, InstallPackages, and RemovePackages, e.g.
> autoremove and allow_deps, although arguably these could be in
> transaction_flags.

The good thing of doing this is that we can actually, do
little break on the client side APIs. I'd suggest the Run()
method, and NO_AUTO_RUN flag, so when the
transaction is set (ie install, remove..) have been called
we call Run().

This could also allow for having a PURGE flag so
a frontend wanting to expose this function can do so
without breaking methods...

BTW allow_deps and autoremove and "PURGE" :D
should be in the flags bitfield to let the remove method
cleaner....

Best.