Fix wIconUpdate logic

Fix wIconUpdate logic

This patch changes the logic for choosing the icon, and gets rid of
wIconChangeImage in favor of wIconChangeImageFile.

Old logic:
* On create, load either NET_WM_ICON or the file from disk.
* On update, choose (in order):
  1. WM_HINTS icon_window
  2. WM_HINTS icon_pixmap
  3. Whatever was loaded on creation, unless wIconChangeImage or
	 wIconChangeImageFile was called.
  4. Default icon.

New logic:
* On update, choose (in order):
  1. WM_HINTS icon_window
  2. NET_WM_ICON
  3. WM_HINTS icon_pixmap
  4. Icon file from disk.
  5. Default icon.

Signed-off-by: Brad Jorsch <anomie@users.sourceforge.net>