TextEdit.textDocument loads according to mime type regardless of TextEdit.textFormat

So we can fix that: QQuickTextDocumentPrivate::load() can check editor->textFormat().

The next problem then is a sort of initialization race condition:

TextEdit {
       textDocument.source: "whatever.md"
       textFormat: TextEdit.PlainText
}

textFormat might not be set yet at the time that QQuickTextDocumentPrivate::load() runs. So it needs to be delayed. A workaround is to set source in Component.onCompleted.