Refactored WMS rendering.

Key features of this pull request are:

- thorough refactoring of the WMS layer selection and of the mapserver layer factories;
- implementation of the DB-stored Vector Masks including the ingestion of the geometry from a file (e.g., GDAL-GML feature collection) or EO-O&M metadata;
- implementation of the generic Key/Value metadata to the EOObject model (including the Admin and CLI UI);
- implementation of the pre-rendered "automatic" WMS views based on the Key/Value metadata;
- improved ordering of the layers rendered by the mapserver.

Things to be done: 
- testing with the current WMS test cases;
- creation of new test case for the newly introduced features;
- update the documentation.