Flint: Display images also on sub scans

Closes #2215 (closed)

This MR allows to display sub plots also for a sequence displaying itself a plot (which is the case for phase contrast sinogram).

Plus the scan status widget was reworked to display a little better the relationship between sequence and subscans.

This change follow to same rules applied for scans containing a scatter description.

• The sequence can provide a plot
  • By description, a plot can be defined to use the default scatter widget or a new widget (there is a plot name)
• A subscan can also provide a plot
  • If the plot is already used by the sequence it is skipped
  • Else it is displayed

IMO too much changes to be backported to 1.6.