Category initializers for MapStore and MapManager

* mxp.plugins.CategoriesInitializer for MapManager
* gxp.plugins.CategoriesInitializer for MapStore

These plugins check if there are a list of categories in GeoStore and create if not present.

Wiki page here: https://github.com/geosolutions-it/MapStore/wiki/Categories-Initializer (for gxp)

Fixes #386.