Add a "delete marker" functionality

[infinity0]

There is no "delete marker" in the "contribute" mode to correct mistakes. The only way to delete one point is to refresh the page without saving, which loses all your other work.

[teleyinex]

Hi,

Thanks for spotting this issue! I've added it to other apps, but I forget to include it in this demo one :-( I'll add it soon!

Cheers,

Daniel