DatasetIOPlugin: revert to using DatasetService

There are significant problems with trying to call another module from
within the DatasetIOPlugin. Let's revert to simpler times when we just
used DatasetService.
Remove OpenDataset interface

Reverts DefaultOpenDataset to OpenDataset, removing the need for
overriding.
SaveDataset: specify non-clashing menu path

We need to add a space character to the end of the path so that it will
not clash with OpenDataset in ImageJ1.
Merge branch 'simplify-open-dataset'

The new open dataset framework was incorrect, as it was nesting command calls
in a way that removed any possibility of control over those executions.

OpenDataset and SaveDataset are now just stand-alone commands, and the
DatasetIOPlugin again defers to the DatasetService as intended.