Convert Free_Pixmap macro into terminal hook

Convert Free_Pixmap macro into terminal hook

* src/termhooks.h (terminal) [HAVE_WINDOW_SYSTEM]: New terminal hook
free_pixmap.

* src/image.c: Replace Free_Pixmap with free_pixmap.

* src/msdos.h (FRAME_X_DISPLAY):
* src/nsgui.h (Display):
* src/nsterm.h (FRAME_NS_DISPLAY, FRAME_X_DISPLAY, FRAME_X_SCREEN)
(FRAME_X_VISUAL):
* src/w32term.h (FRAME_X_DISPLAY): Remove unused X-compatibility
macros and typedefs.

* src/nsterm.m:
* src/w32term.c:
* src/xterm.c: Implement and set free_pixmap hook.