mplayer: remove duplicated mouse autohide check
cocoa_common: remove most of the special handling for cursor autohide

This is mostly related to the fullscreen behaviour. cecbd8864 introduces an
option to make mpv behave like a OSX user would expect. This commit changes
the Cocoa parts of the code to be consistent with the behaviour on X11. Old
behaviour is still available through the option mentioned in cecbd8864.

There is still custom logic in the cocoa backend and it can probably be moved
to core:

  * Don't perform autohide if the mouse is down
  * Don't perform autohide outside of the video window

Fixes #218 (by accident)
input: convert to new msg API