EngDiff: Provide background subtracted data to GSAS

[DanielMurphy22]

Some design discussions needed on this. GSASII can do it's own background fitting, but Saurabh likes being able to control the background within Mantid and trusts out background subtraction. This issue is to provide Background subtracted focussed data for inputting to GSASII. This should be an option (maybe the most popular / common option) but it should also be possible to feed in normal focussed data.

This needs a plan for the intended user flow through the interface.

• Background subtracted data might be outputted from the fitting tab (would this require a new button?)
• Or should the background subtraction table be copied to the GSASII tab or copied to its own tab?

Tasks:

• [x] Move Seqential and Serial fit buttons into fitting plot structure
• [x] Determine how much of the fitting data_model/view/presenter can be moved into common
• [x] Move those elements into the common area
• [ ] Replace the current loader in the GSAS tab with the elements moved into common, so that the table is available on the GSAS tab as in the fitting tab

[DannyHindson]

Based on recent discussions inside the diffraction team and with Saurabh we think the best plan is to add a table widget to the GSAS tab that is similar to the one already on the Fitting tab. Such a table might also support doing serial and sequential fitting across multiple runs on the GSAS tab (but that's a separate change\issue)