Python: add PySide installation check on document open

Checks if the document imports PySide and whether the selected python
interpreter can find a PySide installation. If not show a global info
bar that can install PySide via pip like the python lsp server.

Task-number: PYSIDE-1742
Change-Id: I02c0d5f6eb268f3d8826d4fb9d9ec3c7c48b8638
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/python/pythonsettings.cpp

@@ -549,6 +549,12 @@ Interpreter PythonSettings::defaultInterpreter()
     return interpreterOptionsPage().defaultInterpreter();
 }
 
+Interpreter PythonSettings::interpreter(const QString &interpreterId)
+{
+    const QList<Interpreter> interpreters = PythonSettings::interpreters();
+    return Utils::findOrDefault(interpreters, Utils::equal(&Interpreter::id, interpreterId));
+}
+
 } // namespace Internal
 } // namespace Python