[FEATURE] item_variables expression function inside compositions

[nyalldawson]

This adds a new item_variables expression function when expressions are used inside a composition context.

The function takes a single argument, the id for an item inside the composition, and returns a map of variable name to value for that item.

This allows you to do things like insert text in a label fetching properties of another item in the composition, eg

Insert scale of map into a label:

map_get( item_variables( 'map'),'map_scale')

Insert x coordinate of map center into a label:

x(map_get( item_variables( 'map'),'map_extent_center'))

Previously this has been impossible to do without resorting to custom python expression functions, yet it's a common requirement

[nyalldawson]

Bonus commit:

I've added some new expression variables for map settings (these also work with composer maps, using the item_variables function)

• map_crs, map_crs_definition: retrieves crs details for map
• map_units: retrieves units for map (eg 'meters','degrees')

[rldhont]

Hi @nyalldawson
Do you think map_crs, map_crs_definition and map_units could be backported ?

[nyalldawson]

@rldhont I'd be -0 for that. It'd be backporting a feature, but I won't stand in the way if you want to take responsibility.