MapStore mapfish specification creator refactor

[mbarto]

The payload for printing maps sent to mapfish is created by MapStore in the PrintUtils utility (in the getMapfishPrintSpecification function), as an hardcoded transformation.

To allow customization of the payload, we need the following:

• allow full delegation of getMapfishPrintSpecification to a different (project) function
• refactor actual getMapfishPrintSpecification to decentralize transformer functions from PrintUtils to where they belong (e.g. WMS layer transformation should be moved to the WMS area) and allow plugging new transformers for additional fields
• automatically include in the mapfish payload custom properties set by custom controls (all those properties will be stored in a plugins section of the printing spec, so they will be easily recognized)