Move Cocoa interfaces up

This should let the implementations know which methods each interface
has.
Update GLFW_CURSOR_HIDDEN to only hide the cursor when in the window

This is probably a little more involved than it needs to be, but Cocoa
has to have cursor hide/unhide calls balanced, so it requires a little
more work than just calling hide/unhide when the window is shown and
such.
Update _glfwPlatformSetCursorPos to avoid deprecated method

It'll still use it when the more recent method is unavailable, but
otherwise it now uses the non-deprecated one. Unfortunately, it takes
a rect instead of a point, so it's not very good looking.