QDomDocument: improve the setContent() API

QTBUG-104130 requires adding a way of configuring how the whitespace-only text nodes are handled by QDomDocument::setContent(). The natural way of doing it is adding one more parameter to setContent(). Considering that there are 7 overloads of setContent() already, adding one more parameter will require adding 7 more overloads. If we decide to add other options in future (for example invalid data handling option, see QTBUG-104474), the number of overloads will double, so this approach doesn't scale. A cleaner option would be adding a way of passing multiple parse options to setContent() (e.g. via a new ParseOptions type, as suggested by Marc in comments of https://codereview.qt-project.org/c/qt/qtbase/+/415926). Then we could also handle the namespaceProcessing argument via the parse options.

Other improvements suggested by Marc:

• Introduce a new Result struct for the output parameters (errorMsg, errorLine, errorColumn)

• Use QAnyStringView for the string arguments.