Add on_release message to Slider

[Masterchef365]

Sometimes, it's useful to have a message fire only when the user has finished changing a parameter. Other widgets like text inputs may also benefit from this functionality, and I'd be happy to implement those as well!

[hecrj]

Thanks!

I think this may be a nice addition. Do you have any specific use cases in mind for this?

[Masterchef365]

Incorporated the changes you suggested into it; had to make the message impl Clone but I assume that's not a big deal since Button requires it too. Added a bit of an explanation for the on_release message as well; the use case I have is that a long running task (actually a whole thread) is spawned to perform an update to a large amount of data every time the slider is changed. on_change would fire that event every time a mouse position update was detected, and the result was that my app suddenly used 100% of all cores every time the slider was moved. I could've solved this by limiting when the thread could be spawned, but that wasn't exactly the behavior I wanted.

[hecrj]

Great! Thank you 🎉