Create a method for post-initialization addition of new Actions

[jywarren]

In publiclab/Leaflet.DistortableImage#141 we're discussing how to make our toolbars easier to modify during more complex UI interactions, and would like to be able to add or remove Actions from the toolbar at runtime. Would we be able to run, for example, something like:

toolbar.addAction(...);
toolbar.removeAction(...);

Thanks, love to hear others' thoughts on this!