Simplify handling item-data-source property in elements
They are all the same, let's remove copy-pasted code.
This also makes the property editable for all elements supporting data source, so also completer works there.
+ Remove unnecessary and unused support of static text with $ prefix
Test Plan: Try data source properties of elements in KReportExample and KEXI Reports
Reviewed By: piggz
Differential Revision: https://phabricator.kde.org/D9941