Shortcuts in BALLView are handled globally

[tkemmer]

The shortcut handling of BALLView should be limited to the widget that is currently focused (at least for some shortcuts). Especially high-impact shortcuts like Del, Space, or even Ctrl+{C,V,X} often cause unwanted side effects, e.g., deleting selected structures in the wrong context, preventing spaces from being used or characters from being deleted in HTML form fields (via QtWebEngineView).