Add container for columns

[drzamich]

Because of the Semantic UI styles, column will only have desired width if they are specific children of parents.

Right now only elements that have classes ui grid are dashboard_body() and tab_item(). Sometimes user would want to insert columns somewhere deeper in the UI. We should give them a readymade container for columns that will allow them to do that without hack like adding custom classes to divs.

[dokato]

Then this must be related #152 .