simple sub-windows in lightweightMode

Hi, 

this PR includes a fix to make opening sub-windows again possible on Android (this was broken somehow by https://github.com/jasonkneen/com.jasonkneen.tabgroup/commit/dec4d51db7d65277c450fc3951696c2553bbd542).

I also changed the way sub-windows are opened in lightweightMode for some reasons. I think, it is complicated and not practical to add a custom navigationBar / back button to a sub-window. You can still do this (if you want) or use the ActionBar, just as you like. Instead, you can now open as many sub-windows as you want on a tab and close them with the native back-button (same UX as heavyweightMode).

This is an example how to open another sub-window from within a sub-window (note, that is got a .tab property):
```javascript
var subWin = Alloy.createController('subWindow').getView();
$.subWindow.tab.open(subWin);
```

Let me know what you think about this.