Handle the shortcut key like the meta key

[jkaving]

KeyCombination.shortcut is a way to add platform-independent keyboard shortcuts that will e.g. use "Ctrl" on Windows and "Cmd" on macOS (see the Javadoc in javafx.scene.input.KeyCombination).

This commit just adds a check for getShortcut() and sets the NSEventModifierFlagCommand just like is already done for getMeta().

Fixes #24