fix: lock page scroll when Dialog is open

[chungwong]

It mainly fix two things on dialog with scrollable(long in height) page. The demo didn't show these problems as it is using

1. When the dialog shows, it doesn't prevent the underneath content from scrolling, i.e. when you scroll, the content underneath the backdrop scrolls
2. When the dialog shows or hides, the width of the scrollbar affect the page width and cause the content to shift twice
   a. once when the dialog shows, as the scrollbar is hidden
   b. another when the dialog hides, as the scrollbar is shown.