Add ability to add separate symbols to composer map legend

[qgib]

Author Name: Nikolay Lebedev (Nikolay Lebedev)
Original Redmine Issue: 16857

Redmine category:map_composer/printing

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Now you can only add the whole layer. But there are cases, when you need to add only particular symbols to filtered legend (some symbols not present on a current map extent). For now there is no way to do this.
The workaround is to add the whole layer to the legend and delete unwanted symbols by hand, but that's really hard for 400+ rule-based layers etc.
It could be great to have an "Add symbol" dialog which has an actual layer tree inside (like in QGIS main window) with ability to add any symbol from a tree.