Support Lazy Loading for Large Signals.

[CSSFrancis]

I was going to go through and make this better for large signals so that you could potentially use this as a way to view and check 4D STEM data. That requires lazy loading to be an option.

This also really would benefit from better saving of navigator signals.

To Do:

• virtual imaging (non live/ add compute button)
• better display compute bar to know how long computation should take
• Fix zspy file loading
• Fix 5D sliders being too small on Mac (test on windows)

[ericpre]

(I thought that I already commented here but I must have lost it somehow so I am having another go!)

I am wondering if it would be possible to support reading zspy through the existing "open" action - I guess at the moment, it doesn't work because it is a folder and not a file, but this could be workaround?
This would have the advantage of also easily supporting the drag and drop functionality for zspy?

Regarding loading lazily, would it be worth setting preference setting along a modal dialog that asks what to do the first time being used with the drag and drop functionality?

Can we do separate PRs for the different items of the list, please?