Bring widget system into the core

[sminnee]

Bring the widget system into the cms, so that all pages can use it and not just blogs.

We should let people add widgets to any page, but by the same token, it should be easy to modify that. Add it to SiteTree? but give instructions for how to remove it in your page class, perhaps?

We should update blackcandy to include a widget feed, and we should probably update the blog to make use of the now default widget feed.

[smagnusson]

Given the mass of widgets we now have, it would be ideal to do this. This would appear to be one of the sole new features for the release, and a useful one at that, and I think its worth us having each release have at least one good motivation to show off, something that we can for instance, upload a video about to show neat new things :)

[sminnee]

This is unfortunately a little too complex to get into 2.2.2.

[sminnee]

Let's add them to SiteTree?... This is a change that we've made on a number of individual projects and so it makes sense to incorporate it into the core.