Display theme screenshots in thickbox

At the same time, fixes an issue where active theme screenshot
overflows.
FormHelper: Updating old usage of 'rel' to 'help'
Fix: Deprecated use of `$this->params`

Closes #344
Mark deprecated methods/constants
Merge branch 'master' into 2.0

Conflicts:
	Acl/Controller/AclActionsController.php
	Comments/View/Elements/comments_form.ctp
	Install/Controller/InstallController.php
	Nodes/Controller/NodesController.php
	Nodes/Model/Node.php