improved test case for Database.add_many

check if the undo history is really empty after having called this method
added method Database.remove_many

analogous to add_many, this method only saves one entry in the history,
making it possible to undo removing many entries at once.
added method Database.clear_histories
Merge pull request #918 from derdon/dbstuff

added new Database methods: remove_many and clear_histories