Bugfix: dialogs created during animation was hidden

To reproduce: Close a window with a confirmation dialog while the minimap is
open (eg. gnome-terminal with a running program)

A umapped window is either
1. a window that has not yet been inserted into a space/the scratch layer
2. a window we don't manage (eg. dialogs) meaning its clone is unused and should
   not be shown.

Note that it's not unlikely that we'll start to manage dialog-clones in the
future (#92)

tiling.js

@@ -2077,6 +2077,9 @@ function showHandler(actor) {
     if (Scratch.isScratchWindow(metaWindow))
         return;
 
+    if (metaWindow.unmapped)
+        return;
+
     if (metaWindow.clone.visible || ! onActive || Navigator.navigating
        || inPreview) {
         actor.hide();