[TIMOB-10542] Window modal option

Sample code
```
var win = Titanium.UI.createWindow({
    orientationModes: [Ti.UI.LANDSCAPE_LEFT, Ti.UI.LANDSCAPE_RIGHT, Ti.UI.PORTRAIT]
});

var btn = Ti.UI.createButton({
    title: 'open modal window'
});

btn.addEventListener('click', function(){
    var w = Ti.UI.createWindow();
    var b = Ti.UI.createButton({
    	title: 'close'
    });
    w.add(b);
    b.addEventListener('click', function(){
    	w.close();
    });
    w.open({modal:true})
});

win.add(btn);

win.open();
```