patch: dynamically updated the displayed notification

[TAAPArthur]

Notifications can now have an id and if a notification is already being displayed with that id, instead of creating a new notification, the message displayed will be updated. The implementation is based on message queues.

Depends on #24

[vv52]

How is the message id set in practical use? I've compiled herbe with this port and looked through the C code. I don't see any way to set HERBE_ID with an arg. I've tried running multiple herbe calls from the same process or script and they still queue instead of updating. I might be missing something obvious here but I think basic documentation of this feature would be helpful. Thank you!

PS just wanted to say thank you for your work on this patch, regardless of my ability to intuit it's implementation

[TAAPArthur]

@vv52 I don't actually use herbe anymore, but I believe I let the id be set with the env var HERBE_ID.