Moves sidebar model as a lib.

This seems necessary to further move the plugins/sidebar* that need the sidebar model as libs so we can upgrade to Rails 4.0
Moving sidebars from vendor/plugins to lib to follow Rails 4.0 spcs.
Moves static sidebar from vendor/plugins to lib.
Moves category sidebar from vendor/plugins to lib
Moves xml sidebar from vendor/plugins to lib
Some refactoring so we don't need to create a lib/something_sidebar.rb for every sidebar plugin we create.
Moves the remaining sidebar plugins to lib: