Mms: The "add slide" option will still exsit when reaches max number

When the number of slide shows reaches max number, it still shows
the "Add slide" item.

When the number of slide shows reaches max number, don't show the
"Add slide" item.

BZ-Fixed: BZ1566

Change-Id: Iebedc78f25d92bd437d4d7ff740a20bc5a675474