Only initialise Python once

When switching instances, the Python runner gets another initPython call, despite the previous interpreter existing. Most of the function was either a no-op or was redundant, and the GIL state got confusing. This is the simplest fix.

If things catch fire that I haven't anticipated, instead of returning true, we can destroy the pre-existing interpreter and create a new one by running the rest of the function.

src/runner/pythonrunner.cpp

@@ -1347,6 +1347,8 @@ BOOST_PYTHON_MODULE(moprivate)
 
 bool PythonRunner::initPython(const QString &pythonPath)
 {
+  if (Py_IsInitialized())
+    return true;
   try {
     if (!pythonPath.isEmpty() && !QFile::exists(pythonPath + "/python.exe")) {
       return false;