New: double-clicking in grid mode centers on clicked position

-Made sure pages in grid mode had data-index/-filename attributes

-Changed calls to handleGridDoubleClick to change "this" from the row that was clicked to the page that was clicked

-Replaced handleGridDoubleClick code to just pull in data-index instead of calculating which page was clicked on

-Added zoom proportion/offset calculations to move to double-clicked point

Fixes #222