import_rose.cpp : Align active programming language with model content

- Global `progLang' of type Uml::ProgrammingLanguage::Enum captures
  programming language specified by Rose model.
- In function readAttributes loop of stream.readLine(), if nt is
  PetalNode::nt_string and stringOrNodeOpener is not "(" then test name
  for "language".  If equal then derive progLang from value.string.
- In function loadFromMDL, after parsing the model file,
  - if progLang hash not been established by the model and no code
    generator hash been allocated at UMLApp then allocate generator
    for C++ as default;
  - else if progLang is not the same as the active generator at UMLApp
    then call UMLApp::app()->setGenerator(progLang).

CCBUG: 81364