[feature request] Ability to add custom separators other than date separators

[creepysta]

Description:

Currently there is only date separators in the message_list_options, though there are other separators like banner for {count} Unread Messages. For implementing this, there's nothing exposed in the options class.

There's support for custom message onclick callback functions. It would be great to add a onTapDown to the gesture detector as well, so that the offset for the user touch can be captured and used. This can be considered another feature request.

Finally, the message + timestamp in the message_container can be implemented with a stack instead of a column.

These changes are present in the following PRs
3.10 - #59
3.7 - #58