Re #8290. Only show MaskWorkspaces in combobox.
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.
Re #8290. Guard against the user manually deleting the workspace.
Merge branch 'bugfix/8290_step_scan_bugs' into develop