Fix Step Scan GUI issues found during unscripted testing

[Russell Taylor]

• The pixel mask combobox isn't restricted to the correct workspace type (e.g. it shows table workspaces, which will lead to an uncaught exception if selected).
• The GUI becomes unresponsive if things fail in the runStepScanAlg() method in an unexpected way, as the Qt::WA_TransparentForMouseEvents flag does not get unset.
• The above issue is triggered if the input workspace is deleted manually.
• Similar problems also occur if you delete the table and then try to change the plot.
• With live data, MantidPlot locks up if you happen to hit 'Start' while not in a run (until the run does start).
• If masking the data, hitting 'Start' twice lead to an error:

  AttributeError: 'NoneType' object has no attribute 'setAxisTitle'
       at line 11 in '<Interface>'
  

[Russell Taylor]

Re #8290. Only show MaskWorkspaces in combobox.

Changeset: 26189d8e8aeb74aaeb47965b9f08505e90a15ef3

[Russell Taylor]

Re #8290. Handle setting GUI attributes in an RAII manner.

This ensures that the state is set back to how it should be no matter how the runStepScanAlg method is exited. Previously, the GUI would wind up disabled if an unexpected exception was thrown.

Changeset: 80ee633cf9d99b641725f7bf7760ac824a6584a3

[Russell Taylor]

Re #8290. Guard against the user manually deleting the workspace.

Changeset: 8e698c4d730415409b54f0e5034367bdb2df89d3

[Russell Taylor]

Just a couple more small fixes required - all isolated in the Step Scan GUI class.

[Russell Taylor]

Re #8290. Check table still exists before generating the plot.

Throw up a warning if it's gone missing because the user deleted it manually.

Changeset: 403ecca7b234c86889b4155cc21096c84075f948

[Russell Taylor]

Re #8290. Don't lock up the GUI if the live algorithm takes a while.

In particular, the live algorithm might be waiting for a run to start. This change means that GUI events continue to be processed so that the rest of MantidPlot still functions, and you can close the GUI if you want to.

Changeset: 08b49e9e41d3f2759645ff52dd54571410223d7a

[Russell Taylor]

All items are fixed except the last one, which I couldn't figure out. We'll just have to live with it - at least it doesn't crash anything.

To test: talk to me.

[Andrei Savici]

Merge remote-tracking branch 'origin/bugfix/8290_step_scan_bugs'

Full changeset: ​dcee0d9da8e06210bc75b1149c0d320fdbaf56f1

[Stuart Campbell]

This ticket has been transferred to github issue ​9135