Handle exposure in the widget's expose proc on X

Handle exposure in the widget's expose proc on X

* src/widget.c (emacsFrameClassRec): Don't inherit expose proc.
(get_default_char_pixel_size):
(pixel_to_char_size):
(char_to_pixel_size):
(round_size_to_char):
(EmacsFrameInitialize):
(EmacsFrameRealize):
(EmacsFrameResize): Clean up coding style.
(EmacsFrameExpose): New function.  Expose the frame here to
satisfy the toolkit when it calls the expose proc by hand.

* src/xterm.c (handle_one_xevent): Handle exposure through the
widget instead.