ConfigurationManager editor: Ergonomics improvements. (...)

- The editor no longer supports entering strings.
- Only known plugins and services are known.
- Bugfix: ConfigurationManager was not updated when loading it from XML.
ConfigurationEditor: Added icons. (...)

- Preparing for menu checking boxes.
Lab: Selected configuration is now checked in elements' contex menu.
Bugfix: Deleting a service crashed the program. (...)

- Parent graph was not set when creating ItemViewModel with a service, causing a NRE.
- Moved Description property content of ItemViewModel to ServiceInfo's GetDescriptionOfServiceOrPluginId(string serviceOrPluginId).
Lab: Replaced checkboxes with clear icon buttons
Merge branch 'feature/lab-ergonomy-fixes' into develop