Refs #6540. Beginning to add "time" labeling.

Providing "time" labeling to MDWorkspaces and corresponding reader
plugins. Doesn't seem to work though.
Refs #6540. Attempt to fix issue in reader

Still doesn't work. Sigh. :-(
Refs #6540. Adding label for MDworkspace sources.
Refs #6540. Adding time label to rebinners.
Refs #6540. Changing back to old label getting.
Refs #6540. Adding documentation.
Refs #6540. Adding some tests.
Refs #6540. Add XML hooks to update GUI widget.
Refs #6540. Updating SQWEvent reader for time label.
Refs #6540. Adding tests for file loading presenters.
Refs #6541. Add XML helper objects for MDEventTestHelper data.
(cherry picked from commit 7eaed0aedaf5b226537329a5760c5799c3a80dc8)
Refs #6540. Adding test for MDEW rebinning presenter.
Merge branch '6540_time_axis_labeling' into 6315_build_pv_398

Refs #6315. Fixing merge conflicts.