Adjust loading screen layout

This should make the distinction between loading own files and using the
latest dataset clearer.

The equal height psuedo-columns could be applicable to the chart columns too.

Generic column layout based on:
http://nicolasgallagher.com/multiple-backgrounds-and-borders-with-css2/demo/backgrounds.html
Remove file system message

It was redundant, given the title of using your own datasets anyway.