User interface improvements (side panels, archived layers)

[jb...@google.com]

I have just pushed some significant updates to the Neuroglancer user interface related to side panels:

- Side panels (such as the existing layer controls, and selection details panel) are now highly configurable: they can be docked to the top/left/right/bottom of the viewer, in any combination of rows/columns, and resized in both width and height.  Additionally, the controls for multiple layers can now be shown at once, by using the "pin" icon in the layer side panel title bar.  It is also possible to tear off individual tabs of layer side panels into separate side panels.

- A new "layer list" side panel has been added, as well as support for "archived" layers.  Archived layers are similar to non-visible layers, but are not part of any layer group, and are not shown in the normal layer bars. They are listed only in the new "layer list" side panel, which can be shown using the icon which has been added to the top bar.  The checkboxes at the left indicate which layers are archived.  This feature is expected to be useful in cases where it is desired to include a large number of layers but not clutter the layer bars.  Layers can be restored to the normal unarchived state either by clicking the checkbox (which adds the layer to all layer groups, but leaves it non-visible), or by clicking the visibility toggle icon (which adds the layer to all layer groups and marks it visible), or by dragging it to a specific layer group.

- Hint messages are now shown at the top of the window while dragging layers, layer groups, and side panels, in order to make it clear what the valid drop targets are, what the effect of dropping will be, and what modifier keys can be used.