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'))
