ArMem Viewer: Export data to JSON via context menu

Goal: Right click on any entry in the data part of an instance view allows to export the data at this entry as JSON to the clipboard.

Change InstanceView::prepareTreeContextMenu() [1].

[1] https://gitlab.com/ArmarX/RobotAPI/-/blob/master/source/RobotAPI/libraries/armem_gui/instance/InstanceView.cpp

@fratty Are there already toJson() functions for the aron::core::data types?

@dreher FYI