This was causing an issue if you tried to access widgets in a form using findChildren
widgets could not be casted to QGIs native types
also sipify QgsFilterLineEdit and QgsRelationEditorWidget
Fixes issues with the current approach:
- poor appearance on certain environments (esp OSX)
- large area on spin boxes' right which "swallows" clicks
and blocks interactivity
Fix#12920
Rationale:
- there was a lot of large objects passed by value, so potentially
there's a speed bump from this
- even for implicitly shared classes like QString/QList there's still
a (small) cost for copying the objects when there's no reason to
- it's the right thing to do!