API: pls add mouse position function

This should be easy to add. It will  solve a problem that has been discussed in several posts in Plugin Dev. Implement something akin to the emacs function below. Note that it returns the position without any clicking. (Per legacy, emacs window/frame meanings below are switched.)

mouse-position is a built-in function in `C source code'.

(mouse-position)

Return a list (FRAME X . Y) giving the current mouse frame and position.
The position is given in character cells, where (0, 0) is the
upper-left corner of the frame, X is the horizontal offset, and Y is
the vertical offset.