Dialog improvements

Hi again!

I continued with the dialog improvements, which were part of 2.0.1 release. This time I added the following functionality:

## dialog.reposition(child) function

This function can be invoked in order to reposition the dialog programmatically, e.g. after its contents have been changed by some event. It is used as follows:

    compositionComplete: function(child, parent, context) {
        //Do some stuff
        dialog.reposition(child);
    }

Child argument to the reposition function must be the child view, i.e. the dialog view.

## Limited x-size to window.width()

With very wide content, the dialog will now show x-scroll bar.

## 10 pixel margins

Added 10 pixel margins for a maximum-size dialog (can be used for shadows, etc.). Looks nicer this way.

## Changed MessageBox title and message to use html binding instead of text binding

This is opinionated, but this way, you can add HTML content to the message box, which can be very useful.