added new Database methods: remove_many and clear_histories

Actually I wanted to work on the idea of @dpshelio of being able to split a database into two by a query (see issue #917).

I ended up creating the method ``Database.remove_many`` to remove many database entries "at once" to have only one undo entry for this operation and the method ``Database.clear_histories`` to … clear the undo and redo history.