API Moved "SideBar" widget relation to "widgets" module

Removed BlogTreeExtension in favour of the new WidgetPageExtension
in the "widgets module". Use the following code to update:

Object::add_extension('BlogTree', 'WidgetPageExtension');
Object::add_extension('BlogEntry', 'WidgetPageExtension');

The template placeholder has been renamed from $SideBar to
$SideBarView. In case you're overriding blog templates,
please adjust accordingly.

This change means that widgets can theoretically be applied
to individual blog entries as well, since they need the
"SideBar" relationship as well. In practice, this defaults
to "InheritSideBar" though, which pulls them from the BlogHolder as usual.
Merge pull request #83 from silverstripe/pulls/widget-decoupling

API Moved "SideBar" widget relation to "widgets" module