This adds a new "edit symbol" item to the right-click menu for a
renderer child legend item (eg categories for the categorised
renderer). Selecting it opens a symbol editor dialog which allows
for directly editing the classes symbol. It's much faster than
opening the layer properties and going through the style tab.
The functionality can be used in other renderers too, the only requirement
is that legend nodes need to specify their parent rule key via data().
Note that ayer tree model must have ShowLegendAsTree flag enabled, otherwise
the model will keep showing lists instead of trees.
