Remove EmuWindow

I couldn't find any useful purpose for this class, removing it doesn't seem to break anything.

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 it's own hence the PR.