webadmin: UI plugin dialog API improved

This patch improves the existing UI plugin API for handling dialogs.

(0) Minor dialog / UI plugin infrastructure improvements

(1) Function improved: showDialog

New signature:
    showDialog(title, dialogToken, contentUrl, width, height [, options])

Example usage:
    showDialog('My Dialog', 'my-dialog',
        'http://www.foobar.com/',
        '800px', '600px',
        {
            // Default value = empty array (no buttons)
            buttons: [
                {
                    label: 'Do stuff',
                    onClick: function() {
                        alert('Bump!');
                    }
                }
            ],

            // Default value = false
            resizeEnabled: true,

            // Default value = true
            closeIconVisible: true,

            // Default value = true
            closeOnEscKey: true
        }
    );

The implementation now uses WebAdmin's GWTP dialog infrastructure
via non-singleton DynamicUrlContentPopupPresenterWidget component
so that multiple dynamic dialog instances can be shown at a time
if necessary.

(2) New function: setDialogContentUrl

New signature:
    setDialogContentUrl(dialogToken, contentUrl)

Example usage:
    setDialogContentUrl('my-dialog', 'http://www.example.com/')

(3) New function: closeDialog

New signature:
    closeDialog(dialogToken)

Example usage:
    closeDialog('my-dialog')

Change-Id: I4654a25bed7122c45d432e3860ed710fa4ca3dd9
Signed-off-by: Vojtech Szocs <vszocs@redhat.com>