Reimplement XMLSimpleReader under an ifdef in SVGParser to read white-space only nodes.
Previously, 153fa48b had ported away from using QXmlSimpleReader. However, the new API suggested by Qt doesn't actually support reading white-space only nodes until Qt6. Therefore, we ifdef reading it in the old way for Qt5
To reproduce:
- Create a text with white space inside.
- Select the text before the white-space, color it red.
- Select the text after the white-space, color it green.
- Save
- Load
Without this patch, the white space will have dissapeared at step 5, because the node in question would only contain white space, and by default this gets skipped by QDomDocument.
Needs testing on Qt6.