Workspace classes sorting

[fabjanvucina]

We added a new property to the workspace class model - Display order

We use it as the highest priority value when sorting workspace classes. If two workspace classes have the same display order, we will next take into account their Usage multiplier value. The workspace class with the higher usage multiplier would be displayed first.

Finally, the lowest priority sorting criterion is the alphabetical order of the workspace class name.

The workspace class docs entry needs to be expanded to include this new property. Also, we need an new section which describes the sorting behaviour.