Multiple plots of different WS

[NickDraper]

Alter the user interface to simplify the task of plotting curve from different workspaces on top of each other.
Suggestions:

• Allow multiple workspace selection (ensure all of the right click options are valid), then Plot Spectrum (instead of first spectrum) would ask wich spectrun (as long as there was more than one) and plot that spectrum from all workspaces (any that do not contain that index -> error log and no curve).
• As above but when selecting a WS group
• Dragging a WS onto a graph asks which spectrum and adds a curve
• Dragging an index from a matrix WS to an existing plot will add a curve.
• Any other inspirations

[NickDraper]

This issue was originally trac ticket 950