QUiLoader, QAbstractFormBuilder: Introduce errorString().

QUiLoader, QAbstractFormBuilder: Introduce errorString().

Introduce errorString() to be able to obtain load errors
in Qt Designer. Remove automatic Qt 3 form conversion
from Qt Designer (since uic3 no longer exists) and move
all form load error checking logic including language
check into QAbstractFormBuilder.
Make language accessible in FormBuilderExtra in case
Jambi is revived.

Reviewed-by: hjk