xwm: Add icon support to the frame

This fetches the _NET_WM_ICON property of the X11 window, and use the
first image found as the frame icon.

This has been tested with various X11 programs, and improves usability
and user-friendliness a bit.

Changes since v1:
- Changed frame_button_create() to use
  frame_button_create_from_surface() internally.
- Removed a check that should never have been commited.

Changes since v2:
- Request UINT32_MAX items instead of 2048, to avoid cutting valid
  icons.
- Strengthen checks against malformed input.
- Handle XCB_PROPERTY_DELETE to remove the icon.
- Schedule a repaint if the icon changed.

Changes since v3:
- Keep the previous Cairo surface until the new one has been
  successfully loaded.
- Use uint32_t for cardinals.  Unsigned is the same type except on
  16-bit machines, but uint32_t is clearer.
- Declare length as uint32_t too, like in xcb_get_property_reply_t.

Signed-off-by: Emmanuel Gil Peyrot <linkmauve@linkmauve.fr>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>

clients/window.c

@@ -2546,7 +2546,7 @@ window_frame_create(struct window *window, void *data)
 
 	frame = xzalloc(sizeof *frame);
 	frame->frame = frame_create(window->display->theme, 0, 0,
-				    buttons, window->title);
+	                            buttons, window->title, NULL);
 
 	frame->widget = window_add_widget(window, frame);
 	frame->child = widget_add_widget(frame->widget, data);
@@ -5449,7 +5449,7 @@ create_menu(struct display *display,
 	menu->user_data = user_data;
 	menu->widget = window_add_widget(menu->window, menu);
 	menu->frame = frame_create(window->display->theme, 0, 0,
-				   FRAME_BUTTON_NONE, NULL);
+	                           FRAME_BUTTON_NONE, NULL, NULL);
 	fail_on_null(menu->frame, 0, __FILE__, __LINE__);
 	menu->entries = entries;
 	menu->count = count;