cocoa_common: simplify resize code

This introduces some changes in resize behaviour. Most importantly the window
frame is not constrained to it's screen's `visibleFrame`. Anyone who still wants
that kind of behaviour when opening a video, can use `--autofit-larger`.

Even though the size of the window is not constrained, it's position is, so
that the titlebar will always be visible. When using `--no-border` even the
position will not be constrained in any way.