Add support for modifier keys while dropping

[koppor]

A user can drop a file into the bibliography. Currently, there is a global setting, what should happen: copy, move, rename, ... This is used by users (see JabRef#5546).

Windows Explorer supports modifier keys (Alt, Shift, and Ctrl) when copying a file. See https://www.techjunkie.com/drag-and-drop-copy-move-shortcut-windows/ for details. Ttis was supported up to JabRef 5.0, but removed in JabRef#5554. Reasons were that Linux Desktops, Windows, and Mac OS X behave differently and developers of JabRef saw no value in the modifier keys.

If someone else then me sees a value, he can describe a solution and come up with a PR.

Initial idea:

a) Respective icon is shown while dragging.
b) In case a modifier key is pressed (see the deleted comments: Alt, Ctrl, Shift), the OS-default action w.r.t. that key is taken. For instance, if I press Ctrl on Windows, the file os copied regardless of the FileDragDropPreferenceType.

[koppor]

Case b) does not work. JavaFX currently does not provide an API for that. See https://openjfx.io/javadoc/11/javafx.graphics/javafx/scene/input/DragEvent.html.