[Lens] Allow dragging between layers

[myasonik]

For both keyboard and mouse use, I'd love to be able to drag between layers.

This won't be possible if they're different indexes, I guess, but I think should otherwise work.

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[ghudgins]

another angle on why this is important - it's the only way to get a mixed XY. would be nice as an option if the user has the same index pattern on the new panel (which is the default and definitely is useful for larger index patterns)

[flash1293]

An important use case for this is to copy a complex filters dimension to use it in multiple places.