WMS Feature info plugin for MapStore.

* Dependencies gxp/FeatureEditorGrid.js, gxp/FormFieldHelp.js and gxp/SchemaAnnotations.js has been added.
* Uses feature editor grid 'readonly' to show feature info in an accordeon. Fixes #91 issue.
* Closes feature info popup when the widget has been deactivated. Fixes #178 issue.

This widget has been added in ux/MapStore directory and build it in cfg file in a new asset called 'mapstore.js'.