Added highdpi unity window controls.

I've replaced the window control PNGs with SVGs to support highdpi display. I've also used symlinks instead of copies for the icons that should look the same (e.g. `close.svg` and `close_focused_normal.svg`).

You should check, if I've used the correct colors:
- focused normal: `#DCDCDC`
- (un)focused prelight: `#d64937`
- (un)focused pressed: `#d64937` with opacity `0.75`
- unfocused normal: `#AAAAAA`