First try on improving metadata loading.

This is a "staging" branch. It will not be merged "as is"

I was trying to improve the magnet metadata loading in the AddNewTorrent Dialogm, but I have hit a hard wall. The ui freezes either on the "retrieving metadata..." stage or in the "metadata retrieval complete" stage. I tried to debug this but I cannot find where the freeze happens. I need more eyes on this.

Basically this patch should resolve most of #894 issues. This is the **ONLY** thing left for 3.1.0. It is a blocking bug. (all others will be moved for 3.1.1).