[Lens] Drag and drop handling

[flash1293]

• Extend APIs where necessary
• Registers global context for drag and drop handling
• Add drag and drop for indexpattern datasource

Has to work without assuming a single react tree (because of agnostic rendering)

[elasticmachine]

Pinging @elastic/kibana-app

[chrisdavies]

Wylie and I discussed a bit yesterday, and decided to implement the most basic feature set we need and nothing more. So, the current PR does not provide out-of-the-box mechanisms for re-ordering. It's just drag / drop / change some CSS classes if the drop target is active or supports the dragged item.

[chrisdavies]

As noted by Joe, we'll need to look into cross-platform issues, such as drag and drop on mobile.

There are some polyfills available which we can investigate when the time comes.

[flash1293]

@chrisdavies I think we can add a separate :Lens issue for that

[chrisdavies]

I think this is addressed well enough by the PR mentioned above. Closing for now.