[FEATURE] Legend filtering based on map content (in main window, composer, WMS)

There is new "filter" button in layers panel that toggles this functionality
and in composer legend widget.

Related feature is that layer tree now shows symbols in map units with correct size
(even when filtering is not enabled) so as the map view changes the legend node icons
are updated too (if they use map units).

GetLegendGraphics in WMS server
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This is an extension of standard GetLegendGraphics request according to MapServer RFC 101.
See the document for more details: http://mapserver.org/development/rfc/ms-rfc-101.html

In summary, clients need to add BBOX and CRS/SRS parameters to get appropriate legend based on the given map view.
Parameters WIDTH and HEIGHT are also taken into account as they specify map view image size for correct calculation
of size of legend symbols (if they are based on map units).

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This software has been commissioned by Tuscany Region (Italy),
co-funded by the European Commission and developed under the project LIFE12 ENV/IT/001054 LIFE + IMAGINE.
The software has been realized by Gis3W s.a.s.

Questo software è stato commissionato da Regione Toscana (Italia),
cofinanziato dalla Commissione Europea e sviluppato nell'ambito del progetto LIFE12 ENV/IT/001054 LIFE + IMAGINE.
Il software è stato realizzato da Gis3W s.a.s.