WIP: More drag and drop support
WIP: Drag and drop support for frame with 2 views.
WIP: Drag and Drop: fix single view case, add support for adding at end of compatible container.
WIP: Finish drag and drop support into top level frame anchors.
Fix bug where splitters would not "re-connect" to the right siblings when layout changes.
Code cleanup.
Make anchor smaller, experiment with showing anchors on containers.
WIP: Drag & Drop works for views.

* Dynamically show right anchor as cursor moves around view borders.
* Implement drop into a view at a location ("left", "top", etc.).
Drag-drop: update mouse cursor to "none" or "move" depending on available target.
Update background of anchors.
Refactor drag-drop to work with AXIOM-FRAME container.
Make drop-drop work with containers.
Create custom element for anchors.
Use <axiom-anchors> element.
Code cleanup.
Fix container anchors not displaying.
Code cleanup.