Hugo Mercier d16cdcf449 Add more options for filtering legend elements
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.
2015-11-03 10:39:09 +01:00
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