winwidget.c: Set _NET_WM_NAME and _NET_WM_ICON_NAME, marked as utf8 (closes #44)

Note that feh does not try to convert encodings or anything. It expects
filenames to be in UTF8 and passe them on to NET_WM_NAME and NET_WM_ICON NAME.
The only new thing is that it now also sets _NET_WM_NAME and _NET_WM_ICON_NAME
and explicitly tells X11 that they are in UTF-8. Again, no recode, if you use
UTF-8 filesystems (which you should), everything is fine.