Improvement: automatic column selection

[Jozsef]

Until now when you added a new category group on the Dashboard, the category has been always added to the top of the left column. This resulted a growing left column (if you didn't rearrange the categories, or you kept adding categories from the Add Bookmark dialog with the "+" button).

From now if you add a category (both from the Add Bookmark dialog, and on the Dashboard) it will be added to the top of the shortest column.