Spatial query form fixes

* Export button fixes.
 * Review retrocompatibility for show CSV
 * Allow file export for complex filters
 * New option `exportDoubleCheck`: Flag to make check before export
 * Fixes IE 8 bug on feature grid plugin (on export action)
 * Fix feature grid layout for IE
 * Select `CSV` as default export for the export window.
* Refactor spatial selectors methods
 * Refactor code of the spatial selector methods as widgets instead plugins to fix #339 issue.
 * Optionally add a geometry operation combo for the spatial filter

This commit is related to those issues:

* fixes #335
* fixes #339
* fixes #345
* fixes #346

Also, I've done some changes on wiki pages in order to harmonize with this code:

* [Gecoder plugin](https://github.com/geosolutions-it/MapStore/wiki/Custom-Geocoder-based-on-WFS)
* [Spatial selector query form plugin](https://github.com/geosolutions-it/MapStore/wiki/SpatialSelectorQueryForm-Plugin)
* [Feature Grid plugin](https://github.com/geosolutions-it/MapStore/wiki/Feature-Grid-Plugin)