Rename generic x_* identifiers

Rename generic x_* identifiers

* src/image.c: Rename x_* procedures to image_*.

* src/frame.c: Rename x_* procedures to gui_*. Rename
xrdb_get_resource to gui_display_get_resource. Rename x_get_arg to
gui_display_get arg.

* src/frame.h: Rename can_x_set_window_size to can_set_window_size.

* src/xfaces.c: Rename realize_x_face to realize_gui_face. Rename
x_supports_face_attributes_p to gui_supports_face_attributes_p.

* src/keyboard.c:
* src/lisp.h:
* src/nsterm.m:
* src/w32term.c:
* src/xterm.c: Rename x_get_keysym_name to get_keysym_name.

* src/nsfns.c:
* src/nsterm.m: Rename x_* procedures to ns_*.

* src/w32fns.c:
* src/w32term.c: Rename x_* procedures to w32_*.

* src/termhooks.h (query_colors, get_focus_frame, focus_frame_hook)
(frame_visible_invisible_hook, iconify_frame_hook)
(set_window_size_hook, set_frame_offset_hook, set_frame_alpha_hook)
(set_new_font_hook, set_bitmap_icon_hook, implicit_set_name_hook)
(activate_menubar_hook, change_tool_bar_height_hook)
(set_scroll_bar_default_width_hook)
(set_scroll_bar_default_height_hook, get_string_resource_hook): New
terminal hooks to replace backend-specific x_* procedures.

* src/dispextern.h (clear_under_internal_border): New RIF procedure.

* src/alloc.c:
* src/frame.c:
* src/xdisp.c: Use FRAME_OUTPUT_DATA instead of FRAME_X_OUTPUT.

* src/frame.c:
* src/w32term.c:
* src/w32fns.c: Use FRAME_NATIVE_WINDOW instead of FRAME_X_WINDOW.