Refactor mouse positioning stuff to avoid code duplication.

Refactor mouse positioning stuff to avoid code duplication.
* frame.h (frame_char_to_pixel_position): New function.
(x_set_mouse_position): Rename to...
(frame_set_mouse_position): ...new function.
(frame_set_mouse_pixel_position): Add prototype.
* nsterm.m, w32term.c, xterm.c (x_set_mouse_pixel_position):
Rename to frame_set_mouse_pixel_position.
* frame.c (Fset_mouse_pixel_position, Fset_mouse_position):
Adjust users.
* xterm.h, w32term.h ( x_set_mouse_position)
(x_set_mouse_pixel_position): Remove prototypes.