For the c++ api dox this expands to "\c nullptr" (the
\c directive indicates a code literal value), and for sipify/Python
it expands to ``None`` (`` is sphinx annotation for literal values)
Makes for nicer dox for both c++ and Python!
instead of QgsAttributeForm - because it's used on places not including QgsAttributeForm
now it's able to use @form_mode in expressions of containers and QML widgets
Removed QmlExpression from api
On activation of template box, we overwrite the text in the GUI
The preview is loaded including expression values on start
Takes expression instead of currentText from ExpressionWidget - means it delivers fields in quotes
