added OnStartDragListener

Hi,

in my project I needed to know not only that the menu is opening, opened, closing or closed, but when the user starts dragging the menu (to open it or to close it ). So I've created `OnStartDragListener` which calls `onStartDrag(boolean isMenuShowing)`. I thought that it could be usefull for others.

I've also made a small correction in javadocs.