Fix issue #68: (re)enable auto saving of StudySetup to JSON and add ability to load specific JSON setup file

Closes #68 Here:

1. automatic saving to JSON when any property of the StudySetup actor is changed is enabled
2. the dedicated save button is consequently removed (as everything is saved automatically)
3. it is made possible to load specific JSON files with different setups using a file picker window. The paths are stored relative to the [ProjectDir]/StudyFramework folder (Now built with UE's platform independent functions)

Because we are using relative paths, it is only possible to select files that are stored on the same drive as the project folder. I have added an error message, when the conversion to a relative path fails (selecting a file on a different drive is a likely cause for it failing) and also mentioned this in the wiki.