Nyall Dawson d70b60d48f [FEATURE][layouts] Allow expressions to be embedded inside legend item text
This feature allows for expressions to be embedded directly inside
legend item text (e.g. group, subgroup and item text). The expressions
are evaluated at render time, with full knowledge of the legend's
expression context (so can utilise variables from the layout/layout item/
etc)

There's no UI for this yet (that will come in 3.8), but expressions are
entered using the standard [% 1 + 2 %] format.

E.g. a legend item text of

    My layer (rendered at 1:[% @map_scale %])

will show in the output as

    My layer (renderer at 1:1000)
2019-01-18 20:36:30 +11:00
..