[FEATURE][QGIS-Server] Legend filtering based on map in GetPrint Request

Feature funded by Tecnostudi Ambiente, Faunalia and Andromede-oceanologie.

The legend in composition was fixed and did not represent the layers in the map.
With the work made by @wonder-sk on layer-tree and QgsComposerLegend users will be able
 to configure composer legend as based on all the project layer tree with auto-update
 model or filtered by map.
This commit reused these two QgsComposerLegend's properties to filter it based on map in
 GetPrint Request

The issue #4003 qgis_mapserver getPrint legend options can be closed.
Merge pull request #1593 from rldhont/qgis-server-print-legend

[FEATURE][QGIS-Server] Legend filtering based on map in GetPrint Request