Add a drag-features example

[elemoine]

This PR builds on #3032 to add a "drag-features" example. The example can be tested at http://erilem.net/ol3/drag-features/examples/drag-features.html.

The PR also

• adds handleDownEvent, handleDragEvent, handleMoveEvent and handleUpEvent options to ol.interaction.Pointer. In particular, these options make it possible to create "drag" type custom interactions.
• changes the default implementation of shouldStopEvent. This is to avoid adding another option (shouldStopEvent or similar) to ol.interaction.Pointer.
• adds an ol.Map#getTargetElement method. getTarget may return an Element or a string, depending on what was passed to setTarget. Using getTargetElement you are guaranteed to get an Element instance.
• adds ol.geom.flat.transform.translate and ol.geom.Geometry#translate for translating geometries.

Please review.

Fixes #2947.

[elemoine]

Thanks for your comments @tsauerwein. They are addressed.

[tschaub]

Could this be:

ol.interaction.Pointer.call(this, app.Drag.prototype);

?

[elemoine]

Yes, but with a typecast to make the compiler happy when we compile build/examples/all.combined.js.

[elemoine]

I changed the implementation of ol.geom.flat.transform.translate as we discussed. I think it looks better now. I also removed the changes I had to make to transform2D.

[elemoine]

I'll merge this tomorrow if there's no other feedback. Thanks!

[tschaub]

I appreciate the work you've done to make the library more extensible @elemoine, and I think it would be good to merge this.

When we're all back, I would like to talk a bit more about it - I've got some questions, but nothing that should block.

[elemoine]

Thanks @tschaub. Looking forward to more discussions on this.

[adube]

This example is really neat! I'm going to implement this in a custom control I'm currently building. Thanks a lot for your work on this, @elemoine.

A quick question: any reason why this was not made an official ol3 interaction ?

More about this

In the application I'm currently working on, I use ol3 as a sibling to my code. It also implements the exact same architecture (for custom controls, interactions, etc.) So, I'm going to implement the drag interaction as an 'official one' in my app, which means it wouldn't take too much efforts afterwards to port this to ol3 instead. If someone thinks that this interaction could/should go in ol3 src instead, then I'd be willing to make a PR for this.

Thanks a lot

[elemoine]

There are many interactions we could add, and I am not sure where we should stop. To me the most important is make it possible to write custom interactions.

[adube]

I understand. In my case, I'm perfectly comfortable in customizing it as I please, so everything's fine the way it currently is. Thanks.