[0.11] Support to export entity tree

[Severino]

Adds an export function to the Entity tree's context menu.
This will export the selected item and all it's children.

This will create either a csv file if all entities are of the same type.
Or a zip file containing one csv file each for every entity type.
This export may be used to transfer data from one spacialist instance to another.

Note: This does not export any data models or thesaurus concepts.