RequestHandler#handleAction now exists. It takes the request, and
the action to call on itself. All calls from handleRequest to call an action
will go through this method
Controller#handleAction has had it's signature changed to
match new RequestHandler#handleAction
RequestHandler#findAction has been added, which extracts the
"match URL to rules to find action" portion of RequestHandler#handleRequest
into a separate, overrideable function
GridField#handleAction has beeen renamed to handleAlterAction and
CMSBatchActionHandler#handleAction has been renamed to handleBatchAction to
avoid name clash with new RequestHandler#handleAction
Reason for change: The exact behaviour of request handling depended heavily
on whether you inherited from RequestHandler or Controller, and whether the
rule extracted it's action directly (like "foo/$ID" => 'foo') or dynamically
(like "$Action/$ID" => "handleAction"). This cleans up behaviour so
all calls follow the same path through handleRequest and handleAction, and
the additional behaviour that Controller adds is clear.