klighd: implemented zooming strategy 'ZOOM_TO_FIT_CONTENT' as demanded in KIPRA-1976
* integrated the zooming strategy in action execution, ...
* outsourced bounding box computation from 'DiagramZoomController' into a dedicated class and extended implementation
* added dedicated test class 'DiagramZoomControllerBoundsComputerTest' and a corresponding launch config
* added additional constructor in 'DiagramController' simplifying the application of defaults from the viewContext
* updated related implementations (PiccoloViewer, AbstractOffscreenRenderer) and existing tests