Separate (mostly complete) backend refactoring and DragLayerMixin from (work in progress) backends

[gaearon]

We need to bring code in backend-mousemove branch up to date with changes from 1.0 branch. Currently they're diverged. This involves some manual merging between DragDropMixin (from 1.0) and createDragDropMixin (from backend-mousemove).

After this is done, I suggest we extract a separate branch that contains parts of backend-mousemove not related to MouseMove/TouchMove backends. This should include: potential support for these backends via createDragDropMixin(backend) and backends\HTML5, some new helpers in utils.

This should also include DragFeedbackMixin (let's rename it to DragLayerMixin) and drag-around-experimental example, but without MouseDragDropMixin—let it use HTML5 backend instead (as it actually did before). This would be our only new example in this branch.

The goal is to have custom rendering (via DragFeedbackMixin) and backend refactoring for 1.0.

When this is done, we will need to fix this TODO and merge the result into 1.0.

Next steps would be:

• adding getKey (Track drag source though unmount #64) and proper resurfacing support;
• implementing actual mouse/touch backends.

[gaearon]

Done and released as 0.8.0