Fix build madness

Ok, so somehow the demos weren't set to build by default.

I can't even comprehend how we could debug Mahapps.Metro with the demos not building when pressing F5 oO

If anyone can explain how this worked before, please do it, because I can't even

EDIT: ok, seems like `Mixed Platforms` had everything set. Anyways, this PR cleans the build config up and sets everything to `AnyCPU`