This introduces two new options to filter legend elements:
- filter by expression: a boolean expression can be set. Only symbols of
features that make the expression evaluated to true will be kept in the legend
- filter by polygon: only symbols of features that are inside the given
polygon will be part of the legend.
The polygon filtering is used in particular for a new option in the
composer legend that allows to filter out anything that is not included
in the current atlas polygon.
