Apparently the spec allows to set the _NET_STARTUP_ID value on the property that
WM_CLIENT_LEADER points to instead of the window itself. Thus, if we don't find
a _NET_STARTUP_ID on the window itself, check again on the client leader window.
Apparently GTK even does this (for whatever reason...)...
Signed-off-by: Uli Schlachter <psychon@znc.in>