fix(Grid): Fix a few mobile-browser issues

1. Allow 0-sized scrollbars, which pertains to Android and iOS in my
testing. The scrollbar simply doesn't get displayed. This prevents their
weird horizontal offset issue. The downside is that there's no scrollbar
but so far I can't find a way around it.

2. Fix header cell touch events. Sorting by touch should work, as well as
showing the menu by long-clicking on the header cell.

What still remains is all the other touch events (row selection, header
cell menu button, etc).