Remove EmuWindow

I couldn't find any useful purpose for this class, removing it doesn't seem to break anything.

The most important change to review in this PR is how the window handle is passed around between classes, particularly [this instance](https://github.com/Armada651/dolphin/compare/dolphin-emu:master...master#diff-46b0d67ddedc0ec964937c76aae55ae9R1463).

This change has been done in preparation of exclusive fullscreen support, but makes sense on its own hence the PR.