Allow the terminal display's host to decide whether shortcut keypresses which are also valid terminal key sequences should be overridden and sent to the terminal instead.

Default behaviour in the standalone Konsole is never to override shortcuts. Default behaviour in the terminal part is to always override shortcuts.

Shortcuts which are not valid terminal key sequences (eg. because they have more than two modifiers) are never overridden.

Add an overrideShortcut() signal to the Part to expose this functionality to clients.