Clicking while holding shift now selects a ThumbCell and allows the
multiple ThumbCells at one. The actions in the context menu are applied
to all selected models. "copy url" is disabled, if multiple models are
selected, because that doesn't make sense.
On startup check if the application is running on the OpenJDK JVM. If that is the case,
check that JavaFX is available (because OpenJDK comes without JavaFX, it has to be installed
separately). If it is not available, log an error what to do and exit.
0xboobface