This patch fixes a few issues:
- it was an extension on a CoroutineScope, but that was quite misleading
since the Main dispatcher would be always used regardless of what dispatcher
the owning CoroutineScope was configured with.
- timing was reliant on exact value of the undocumented Snackbar.LENGTH_LONG duration
- coroutine cancellation relied on cooperation of the 'operation' suspend function,
which we can't depend on
New 'allowUndo' fully controls its timing, doesn't imply a dispatcher to its consumers,
and doesn't rely on cooperation of passed-in suspend blocks for cancellation to work.
Colin Lee