nativeWindowPlacement: Adjust to overview changes

title._spacing is no longer defined, so we end up with bogus positions
when window-captions-on-top is set to true. Adjust the positioning to
do without that for now, though the whole extension could use a rewrite
to not copy everything-and-the-kitching-sink, or be killed off as yet
another extension from the original random collection that turned out
too expensive to keep dragging along ...

https://bugzilla.gnome.org/show_bug.cgi?id=787604

extensions/native-window-placement/extension.js

@@ -431,8 +431,8 @@ function enable() {
             let titleX = cloneX + (cloneWidth - titleWidth) / 2;
 
             /// this is the actual difference to original gnome-shell:
-            //let titleY = cloneY + cloneHeight + title._spacing;
-            let titleY = cloneY - title.height + title._spacing;
+            //let titleY = cloneY + cloneHeight - (title.height - this.borderSize) / 2;
+            let titleY = cloneY - (title.height - this.borderSize) / 2;
 
             if (animate)
                 this._animateOverlayActor(title, Math.floor(titleX), Math.floor(titleY), titleWidth);