Support loading of XMI from https://www.softwareideas.net

umbrello/umldoc.cpp
- In function encoding(QIODevice&) while-loop case
  node.isProcessingInstruction() case (pos >= 0), compare encData
  to "UTF-8", "windows-1252" case insensitively.
- In function loadFromXMI case outerTag "xmi:XMI" loop over nodes,
  handle outerTag "packagedElement" alongside "Model" and "Package".

Thanks to Jess Jones for https://bugs.kde.org/attachment.cgi?id=146297

CCBUG: 56184