QKeySequenceEdit: make it configurable to be able to use Esc as part of the sequence

Some applications seem to allow Esc to be part of a shortcut combination.

Currently, it is hard-coded in QKeySequenceEdit to always mean to leave the recording, effectively cancelling it.

I feel like that this ought to be configurable for the application developer using Qt to decide oneself what makes the most sense in the particular application context.

We could leave the default behaviour as is, but add a new property to opt-in for the behaviour that Esc does not cancel the current recording session - rather, it becomes part of the shortcut for the current action.