Observer pattern uses has the data widget as an observable that sends
the plotting widget the workspaces to add or remove from the plot when
the 3rd column in the table is ticked and unticked respectively. The
pattern also handles the removal of indiviual workspaces or the
clearing of all tracked workspaces.

Co-Authored-By: PhilColebrooke <52415735+PhilColebrooke@users.noreply.github.com>

Python 2 has a different method of sorting its dictionaries, so the
comparisons were failing after iterating through them.

Makes the loading process use run numbers if they are available. Falling
back to using the filename to collect data and finally using the
workspace name as is.
Also changes the finder to encourage the use of nexus files.

Originally simply removed from the table, but then was instantly
repopulated if anything else was loaded. Now uses a bi-directional dict
to remove workspace names from their row numbers.