Added curve drawing/editing support

[qpincon]

As discussed in #872.

If the Leaflet.curve plugin is defined, the drawing of curve is enabled (with editing, dragging, rotation).
Touch support is also handled, however not tested extensively.

Limitations:
At the moment there is not self-intersection detection for curves, and cutting the shape with the cut tool is not possible. It is a possible improvement in the future.

Let me know if you have any question.

[Falke-Design]

Thanks for this nice feature! I have spoken with @codeofsumit and we decided that it makes sense to add this as a plugin, because it increases the package size over 100kb and we want that Leaflet.curve is added in the dependencies of package.json (so that it is working without that the user needs to include external Javascript)

If it is ok for you, we will setup a project leaflet-geoman-curve in the geoman-io organisation and then you can create a PR for the new repo.

[qpincon]

Sure, no problem with that!

[Falke-Design]

@qpincon new repo is created, can you please create a PR there: https://github.com/geoman-io/leaflet-geoman-curve

[qpincon]

Thank you!
I had to do some special case implementation for the handling of the rotation/dragging in the source code of those functions however. Should I Leave them here, or should I find a way to integrate them into the plugin you think?

[Falke-Design]

Can we do it in following steps:

1. Make it possible to draw / edit
2. Make it possible to drag, rotate, ...

And I want to look where we need to implement API interfaces for plugins. So let us start with draw and edit ok?

[AndrejGajdos]

I would love to use this funcionality.

@qpincon I tried to run the demo of your fork and there is the button for drawinf curves, but it doesn't work.

@Falke-Design I guess this plugin wasn't implemented. I tried the demo in development branch in https://github.com/geoman-io/leaflet-geoman-curve but it doesn't work.

[Falke-Design]

@AndrejGajdos we currently stopped and never finished the development of leaflet-geoman-curve