QXmlSimpleReader: Extract Method parseNestedFrames()

This helper contains the common code from parseElement() and
parseContent().

The tryUnwindParseStack() function is expanded again, because
the common method now has some type-specific code.

The type-specific code is extracted into local helper lambdas
frameTypeToFunc(), enterFrame(), and stepFrame().

Task-number: QTBUG-149171
Pick-to: 6.12 6.12.0 6.11 6.11.3 6.8 6.5 5.15
Change-Id: If6704d990e79c33516f0278718cea83844d81de5
Reviewed-by: Marc Mutz <marc.mutz@qt.io>