layout: Always allow hiding the overview

When a fullscreen application is focused,
toggling the overview via hot-corner is disabled,
even when the overview is currently visible.
This only makes sense, when the overview is
hidden to not to disturb the behaviour of the
fullscreen application, but leaves an
inconsistency when the overview is visible since
it should work there like when a non-fullscreen-
application is focused.

So, always allow hiding the overview using the
hot corner when the overview is visible.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/429

js/ui/layout.js

@@ -1237,7 +1237,7 @@ var HotCorner = new Lang.Class({
     },
 
     _toggleOverview() {
-        if (this._monitor.inFullscreen)
+        if (this._monitor.inFullscreen && !Main.overview.visible)
             return;
 
         if (Main.overview.shouldToggleByCornerOrButton()) {