Entry point for registering widgets

[kevin]

Need to define a setuptools entry point for registering modules of widgets. This will allow the Toolbox to display all widgets in the system.

[ianb@…]

Some open issues I can think of:

• What do the entry point names mean? Can widgets be looked up by name alone? This can save an import sometimes, but has namespace sharing issues (or you can allow fully-qualified Package#name in addition to name alone). Or you can ignore names.

• Do entry points describe one widget, or a set of widgets?

• Widgets should have groups; so if they are individually enumerated they should still have some attribute that allows you to group them.

• Maybe groups and packages are orthogonal, as a package might add something to a group. I imagine groups like... oh, I dunno. I think there's groups in there somewhere.

Widgets should also have all the metadata needed to describe them; documentation, available options, etc. That makes browsing useful.

[Splee]

Entry points are described  here.

[kevin]

This entry point is very simple. It's just a module or set of modules that need importing. WidgetDescriptions? are automatically registered as soon as they're created, so just importing the module is enough.

[alberto]

committed at r799