Improve initialization of properties in Report Designer

Summary: Improve initialization of KReportSectionData and KReportDesigner, initially clear "modified" flags for properties.

Test Plan:
Insert some elements in kreportexample or Kexi. Or open saved report in Kexi. Properties should not be marked as modified initially.

Also the recent KProperty API 3.0.91 allows for live updates of properties (for now only works for the line element) but it looks like a good start.

Reviewers: piggz

Reviewed By: piggz

Tags: KReport

Differential Revision: https://phabricator.kde.org/D8518