Implement smart popup_during_fullscreen mode

With this commit, the default behavior is to display popups while there
is a fullscreen application only if the popup belongs to that
application (as determined by the WM_TRANSIENT_FOR hint which
applications have to set properly).

fixes #663

src/click.c

@@ -229,7 +229,7 @@ static int route_click(Con *con, xcb_button_press_event_t *event, const bool mod
     /* 3: For floating containers, we also want to raise them on click.
      * We will skip handling events on floating cons in fullscreen mode */
     Con *fs = (ws ? con_get_fullscreen_con(ws, CF_OUTPUT) : NULL);
-    if (floatingcon != NULL && fs == NULL) {
+    if (floatingcon != NULL && fs != con) {
         floating_raise_con(floatingcon);
 
         /* 4: floating_modifier plus left mouse button drags */