[FEATURE] Content based legend in QGIS application

There is new button in layers panel that toggles this functionality
Store in project whether legend filtering by map content is turned on
Rename role (+add docs) for legacy rule key role
[FEATURE] Resizing of symbol preview icons in layer tree when using map units
[FEATURE] Content based legend in composer

There is a new filter button in composer legend properties widget to enable this feature
Merge branch 'wms-content-based-legend' of https://github.com/wonder-sk/QGIS into wms_legend