Push dragend event to overlay

[mstone121]

Currently, it's possible to listen to dragend for draggable edit handles like so:

layer.on('dragend', handleDragEditHandle)

There is, however, no way to listen to dragend in the event the whole layer is dragged. This PR adds this functionality by firing the layer dragend event when the layer's dragging property fires its dragend event.

Closes #1016

---

Make sure these boxes are checked before your pull request (PR) is ready to be reviewed and merged. Thanks!

• PR is descriptively titled 📑 and links the original issue above 🔗
• tests pass -- look for a green checkbox ✔️ a few minutes after opening your PR -- or run tests locally with grunt test
• code is in uniquely-named feature branch and has no merge conflicts 📁
• screenshots/GIFs are attached 📎 in case of UI updates N/A, not a UI update
• @mention the original creator of the issue in a comment below for help or for a review N/A, I created it.

We're happy to help you get this ready -- don't be afraid to ask for help, and don't be discouraged if your tests fail at first!

If tests do fail, click on the red X to learn why by reading the logs.

Please be sure you've reviewed our contribution guidelines at https://publiclab.org/contributing-to-public-lab-software

Thanks!

[welcome]

Thanks for opening this pull request! Dangerbot will test out your code and reply in a bit with some pointers and requests.
There may be some errors, but don't worry! We're here to help! 👍🎉😄

[gitpod-io]

[jywarren]

Thank you very much!!!

[welcome]

Congrats on merging your first pull request! 🙌🎉⚡️
Your code will likely be published to https://mapknitter.org in the next few days.
In the meantime, can you tell us your Twitter handle so we can thank you properly?
Now that you've completed this, you can help someone else take their first step!
See: Public Lab's coding community!