Custom search service editor plugin

[offtherailz]

A plugin should allow administrators to edit the search services used for the current map.

The user should be able to add some WFS services, setting the layer and the attributes to query, and other options, as in the search plugin configuration http://mapstore2.geo-solutions.it/mapstore/docs/api/plugins#plugins.Search

The search services can be saved for the current map. The admin can also choose to completely override the default search services (nominatim or custom defined) for the current map.