Support default keys for makeToolButton

[kaikue]

This is part of the seung-lab merge project (see #340).

It would be convenient to have default keybinds for operations like merge and split, so that the user doesn't have to set them manually. This PR adds an optional defaultKey parameter to the ToolBindingWidget constructor and makeToolButton function, which will fill in the tool button with that key the first time Neuroglancer is loaded, but respect the user's choice and use a different key if one has been set manually.

Caveat: If the user unsets the tool (by double-clicking) then reloads Neuroglancer, it will be set back to the default. The LayerToolBinder.jsonToKey isn't structured to support tool -> undefined mappings, so I'm not sure if there's a good way around this, but it's a minor annoyance.

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[kaikue]

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[jbms]

To summarize the discussion I had with @sdorkenw last week:

1. Having to press and hold shift+key is less convenient than just using a single key. The reason these "tool bindings" use shift+key is to ensure user-customizable bindings never conflict with the built-in key bindings in Neuroglancer. This issue seemed to be lower priority, though.
2. It is inconvenient for users to have to define keys for split and merge actions manually. It would be better if users could just add a graphene datasource and immediately split and merge with a default key binding.
3. In the case that there are multiple layers with graphene data sources, the default key bindings would naturally conflict. Currently user-defined key bindings are not allowed to conflict. Instead, it could be allowed for user-defined key bindings to conflict, with the selected layer taking priority. The selected layer is shown with green border in layer bar, and also the side panel for the selected layer is shown with a green title bar. Note: With the introduction of user-customization of key bindings and the ability to show multiple side panels at once, I had intended to reduce the importance of the selected layer.
4. Alternatively, in addition to the user-customizable key bindings for split and merge, there could be user-non-customizable key bindings for split and merge (not using shift), that only apply to the current selected layer. This would basically amount to supporting the seung lab's existing way of performing split/merge actions in addition to supporting the user-customizable bindings. Having two different ways of performing split/merge actions may be somewhat confusing, though, and would add some unfortunate additional complexity.

This pull request seems to be intended to address point 2 alone. Does the seung lab believe that will be sufficient to address this issue, or is it desired that additional changes also be made?

[jbms]

As far as the implementation, although I can see that makeToolBinding provides a convenient place to add the logic, it is not really the right place, since that will only be called when the UI control is created, i.e. when the side panel is shown.

Instead, the assignment of a default key binding should occur when the layer is created, or when the graph source is first added, I think. But that is indeed a bit tricky.

Also we do really need to solve the issue of being able to unbind the key and not have it come right back.