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>

libweston/compositor-wayland.c

@@ -869,7 +869,7 @@ wayland_output_set_windowed(struct wayland_output *output)
 			return -1;
 	}
 	output->frame = frame_create(b->theme, 100, 100,
-				     FRAME_BUTTON_CLOSE, output->title);
+	                             FRAME_BUTTON_CLOSE, output->title, NULL);
 	if (!output->frame)
 		return -1;