Only subplots appear in the Window menu

[butlerpd]

Describe the bug
This may or may not be the original intent, but if it is, that is quite confusing and doesn't make a lot of sense.
The top section of the Windows menu applies to all windows (plots or otherwise) but the last section is for plots. However while Minimize and clear applies to all plots, only plots containing at least one subplot data set are listed in the list of plots

Expected behavior
All plots, whether they include subplot data or not should be listed

SasView version (please complete the following information):

• Version: 6.0.0b2

Operating system (please complete the following information):

• OS: Windows 10

Additional context
This was identified during the NIST CNR summer school. However this was true in 5.0.6 so it probably should not be a 6.0.0 item, but am tagging it so to start with as requested for all summer school finds.

[krzywon]

Present in v5.0.6.