Qt: Clean up various includes

I noticed that a bunch of the includes in the preferences code were redundant so I purged them, then I went and purged a whole bunch from elsewhere in the module. I've switched many to forward declarations that could not be just removed, and in a few cases I've moved them to a more suitable location.

I mostly just focussed on the Qt framework includes here; I think it very likely that there are more redundant or non-optimal includes to be discovered.