QXmlSimpleReader: Extract Methods Private::parse{Element,Content}Frame()

These new helpers do the actual parsing without stack unwinding, using
the provided parser state.

For now these functions still have recursion - they call parseElement()
and parseContent() on certain code paths.

This change is created to factor out the tryUnwindStack() and the empty
stack handling out of the actual parsing, simplifying the actual
recursion fix.

Task-number: QTBUG-149171
Pick-to: 6.12 6.12.0 6.11.3 6.8 6.5 5.15
Change-Id: Ib38ab1bb40575d5e4e1801d54cd2080d382fd134
Reviewed-by: Marc Mutz <marc.mutz@qt.io>