Separate sidebar widget from contents (resources)

[timkpaine]

Not as complicated as it sounds, right now a sidebar is 1-1 with the icons and treefinder inside, and the tree finder inside is 1-1 with a backend resource. Construction of all of these is done sequentially and immediately.

We should separate this into a 2 step process:

• create sidebar and treefinder, but with no resource attached
• allow for attaching a resource to an existing sidebar/treefinder

When you add a new resource, you should have the option of attaching it to an existing sidebar. The name of the resource can be added as a "folder" at the root level /, ref #249

Prerequisite for #195
Also should solve #245 if we do not attach the resource until the widget is attached to the dom