Added the QTimeline element

[alikalik9]

QTimeline added into nicegui.elements.

This is my first commit on github ever, so please bear with me, if it's not done properly. I tried following the guideline, but it's a bit overwhelming if you never worked on a public repo before.

[falkoschindler]

Thanks for this PR, @alikalik9!

I looked into it and did quite some changes:

• We don't need a custom Vue component in this case. We can simply use q-timeline from Python and can remove the timeline.js file.
• The docstring was at a strange place. It should be the first line of a class or function.
• add_item wasn't implemented. But it's better anyway to add items (or "entries") via with expressions.
• I don't know what _generate_id was supposed to do. It wasn't needed.

So I basically re-wrote the component, added a pytest and a demo for the documentation. I'm still glad you gave it a try and created this pull request. Now NiceGUI has one more useful feature. 🙂

[alikalik9]

@falkoschindler Thank you for the feedback. I will closely examine the changes you made when I find the time, in order to learn from them. :)