Add helping function to check if mode type is selected

[jensschmeink]

A simple check function is added to GtModeProperty to test if a given GtModeTypeProperty is selected

Description

How Has This Been Tested?

Checklist:

• [ ] A test for the new functionality was added (if applicable).
• [ ] All tests run without failure.
• [ ] The changelog has been extended, if this MR contains important changes from the users's point of view.
• [ ] The new code complies with the GTlab's style guide.
• [ ] New interface methods / functions are exported via EXPORT. Non-interface functions are NOT exported.
• [ ] The number of code quality warnings is not increasing.