Refs #9371 Fix caching bug.

When there is only one row in the reflectometry gui, updating a value on
it does not cause the table view to be updated. This was a caching
related problem. This commit fixes that problem and optimises the
dataChanged signal, so that only items that have been updated need to be
fetched by the table view.
Refs #9371 Tidy up the Refl Gui a little more.
Refs #9371 Refactor out some more magic numbers.
Refs #9371 Strip whitespace in Refl Gui table.