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>