had to reset everything because of trailing whitespaces, tabs, etc. The history of the following commits is lost and included in this commit:
commit 5: Implemented ultrasound device persistence: US devices are available again when the program started another time
commit 4: removed digits from framerate visualization
commit 3: cleaned up method OnClickedViewDevice / deactivate UI elements which should not be accessible
commit 2: added global reinit command after start viewing, so the US image appears immediately
commit 1: cleaned up UI