Invisible workspaces option bug fix

[PranavBahuguna]

Description of work.

As stated in the original issue, creating hidden workspaces then checking/unchecking the "show hidden workspaces" option has no effect on those workspaces:

• If "show hidden workspaces" was already checked, unchecking it doesn't cause hidden workspaces to disappear.
• If "show hidden workspaces" was already unchecked, checking it doesn't cause hidden workspaces to appear.

The work done has fixed this so clicking on this option produces the intended effect.

To test:

• Click 'Load' in the workspaces dock.
• Pick a file and make it hidden by prefixing the output workspace name with __
• There will either be a hidden workspace in the dock or no workspace depending on whether "show hidden workspaces" was already checked or not. Either way, go to View->Preferences->Mantid->Options and check/uncheck "show invisible workspaces". Then click 'Apply' or 'OK'.
• The state of the hidden workspaces should now be altered.

Fixes #6798.

Release notes have been updated.

Reviewer

Please comment on the following (full description):

Code Review

• Is the code of an acceptable quality?
• Does the code conform to the coding standards? Is it well structured with small focussed classes/methods/functions?
• Are there unit/system tests in place? Are the unit tests small and test the a class in isolation?
• If there are changes in the release notes then do they describe the changes appropriately?

Functional Tests

• Do changes function as described? Add comments below that describe the tests performed?
• How do the changes handle unexpected situations, e.g. bad input?
• Has the relevant documentation been added/updated?
• Is user-facing documentation written in a user-friendly manner?
• Has developer documentation been updated if required?
• Does everything look good? Comment with the ship it emoji but don't merge. A member of @mantidproject/gatekeepers will take care of it.

[louisemccann]

Works as expected, looks good

[martyngigg]

@PranavBahuguna @louisemccann It's good that the issue has been fixed. However, it looks to as if the branch was not created from master? There 22 commits and many files touched that don't seem related to this issue at all.

We'll need to rectify this before merging the fixes. @PranavBahuguna I think it might be an idea to start a fresh branch and cherry-pick changes over. Please let me know if you'd like help with this.

[PranavBahuguna]

Closing pull request to create new one containing only changes related to this issue.