Adds Popup autoPanPadding(TopLeft & BottomRight) options. Fixes #1588

Fixes Issue described in #1588.

By analogy with options ```paddingTopLeft/paddingBottomRight``` of ```fitBounds()``` method, the Popup options ```autoPanPaddingTopLeft/autoPanPaddingBottomRight``` was added. Usage autoPanPadding options as a Bounds type is impossible. Also, autoPanPadding  influence for calculating dX/dY was fixed, even when popup fully visible in viewport.