fix occasional crash on dragging and dropping pages in slidesorters

pages go into the cache, and sometimes they get deleted before the
cache gets processed. Remove deleted pages when they go away

Change-Id: I291072a8541f4ca36979e9914975d81cc23a9497
remove unused ctor variant

Change-Id: I0c9afcb5819fc83f75ee8c639de56b788c3d516d
pages with equal Priority and Class getting dropped

Change-Id: Ib053dc4b6e5fb5f01f48c71a4b295a53c0ec6715