Support hook_civicrm_permissions on WordPress

CRM-11925 introduced hook_civicrm_permissions to allow extension-modules to define permissions.

If this feature is by an active extension on WordPress, then we currently display an alert: "Some modules define permissions, but the CMS cannot store them".

Two functions need to be added to CRM_Core_Permission_WordPress: isModulePermissionSupported() and upgradePermissions(). The latter receives a list of all affirmatively defined permissions (across all CiviCRM extensions, including core) and performs any necessary steps to register new permissions and unregister old permissions. (Note: In D6 and D7, this is very minimal because we integrate via hook_permissions; however, a bit more work may be required for other platforms.)