Fixes#21986
plus:
- fix multiple string keys with commas in value relation widget
- more robust JSON and array (un)marshalling
- uniform array representation in value relation widgets
- lot of test coverage
- automatic QVariant type conversions in JSON utils
- nodes() return the list of all nodes
- findNodes<T> returns a list of the nodes matching the class
Also drops the regexp for finding form attrs in the value-relation
expressions.
Other minor fixes as suggested in the PR review.
The value relation widget filter expression can now use two
new functions/variables that have access to the current
values and geometry of the form being edited.
This allows for dynamic filtering (drill-down) as explained
in the crowdfunding page:
https://north-road.com/drill-down-cascading-forms/
The new functions/variables are:
Function:
get_current_form_field_value( 'FIELD_NAME' )
Variable:
@current_form_geometry
