API Make LeftAndMain not use session

LeftAndMain used to store the current "PageID" in the Session, but this
could cause subtile problems when using multiple tabs (since they
share the same session)

This moves the Page ID into the URL, before the action

To support this, LeftAndMain::$url_rule has been removed, and instead there
are three new properties

LeftAndMain::$item_id_format
LeftAndMain::$item_actions
LeftAndMain::$collection_actions

See the doc blocks for explicit instructions on their use

It also (of course) causes the URLs to areas of the admin to change,
which will break hard-coded URLs. You should be using
singleton('LeftAndMainDerivativeClass')->Link($action, $id) instead.