BUG Session namespace sharing for CMS controllers

Ideally we could do this without session, but pragmatically
we still need it, because of the inflexible routing system,
and because of performance considerations.

Example: The tree is lazy loaded via a generic  URL (admin/pages/treeview).
While we could add ?ID=<currentpage> to make the view (more or less) stateless,
it would trigger a full tree reload on every tree navigation action.
Instead, we assume that all "reachable" nodes are already cached,
and simply mark a different one as current. For this to work, we need
shared session state between CMS controllers.

See http://open.silverstripe.org/ticket/7815 for detail.

code/controllers/CMSPageEditController.php

@@ -9,4 +9,6 @@ class CMSPageEditController extends CMSMain {
 	static $url_rule = '/$Action/$ID/$OtherID';
 	static $url_priority = 41;
 	static $required_permission_codes = 'CMS_ACCESS_CMSMain';
+	static $session_namespace = 'CMSMain';
+
 }

code/controllers/CMSPageHistoryController.php

@@ -11,6 +11,7 @@ class CMSPageHistoryController extends CMSMain {
 	static $url_priority = 42;
 	static $menu_title = 'History';
 	static $required_permission_codes = 'CMS_ACCESS_CMSMain';
+	static $session_namespace = 'CMSMain';
 	
 	static $allowed_actions = array(
 		'VersionsForm',

code/controllers/CMSPageSettingsController.php

@@ -9,6 +9,7 @@ class CMSPageSettingsController extends CMSMain {
 	static $url_rule = '/$Action/$ID/$OtherID';
 	static $url_priority = 42;
 	static $required_permission_codes = 'CMS_ACCESS_CMSMain';
+	static $session_namespace = 'CMSMain';
 		
 	function getEditForm($id = null, $fields = null) {
 		$record = $this->getRecord($id ? $id : $this->currentPageID());

code/controllers/CMSPagesController.php

@@ -10,6 +10,7 @@ class CMSPagesController extends CMSMain {
 	static $url_priority = 40;
 	static $menu_title = 'Pages';	
 	static $required_permission_codes = 'CMS_ACCESS_CMSMain';
+	static $session_namespace = 'CMSMain';
 
 	function LinkPreview() {
 		return false;
@@ -22,14 +23,6 @@ class CMSPagesController extends CMSMain {
 		return $this->request->getVar('view');
 	}
 
-	/**
-	 * Doesn't deal with a single record, and we need
-	 * to avoid session state from previous record edits leaking in here.
-	 */
-	public function currentPageID() {
-		return false;
-	}
-
 	public function isCurrentPage(DataObject $record) {
 		return false;
 	}