Upgrade to Qt 4.8.6

I have tested on Windows (other platforms are coming) and verified that it compiles.

This commit comes from applying the diff between 4.8.5 and 4.8.6 on top of PhantomJS's source.
There were some merge conflicts which I worked through by hand (mainly bug fixes that we had already backported).

Checked platforms:
- [x] Windows
- [ ] Linux
- [ ] OS X