two column quickhelp dialog, closes #3895

now that quickhelp is a modal dialog, it makes sense to have it be wider
and have the shortcuts take up two columns, instead of one. This change
makes that possible. See the results here:

![new quickhelp css](http://i.imgur.com/nvRTFJm.png)
Merge pull request #3896 from ivanov/fix-3895

two column quickhelp dialog

fits on one page.

closes #3895