Minimal support for _NET_WM_WINDOW_OPACITY

While we don't provide compositing ourselves, add-on compositors such as
xcompmgr need us to copy the _NET_WM_WINDOW_OPACITY property from the
client window to the frame window.

We can do this easily enough.

src/window.c

@@ -1079,6 +1079,8 @@ WWindow *wManageWindow(WScreen *scr, Window window)
 	wwin->frame->flags.is_client_window_frame = 1;
 	wwin->frame->flags.justification = wPreferences.title_justification;
 
+	wNETWMCheckInitialFrameState(wwin);
+
 	/* setup button images */
 	wWindowUpdateButtonImages(wwin);