silx view VDS and external data

[woutdenolf]

closes #3214

Add support for external data (virtual and raw, nothing to do with external links) to silx view:

• add "Virtual" or "ExtRaw" to the Link column (added tests)
• add "external sources" section to the table view (no tests for this)

Data generated with this script looks like this:

[woutdenolf]

@vallsv Not sure about the name ExtRaw in the Link column. It is external data in non-HDF5 files (raw binary format).

[vallsv]

If you prefer you can add a tooltip, so what it is not anymore ambiguous.

Up to you.

[vallsv]

To me it can be merge as it is. Let me know.

[woutdenolf]

It's ready for me as well. ExtRaw is rarely used. We can add a tooltip later if requested.

[vallsv]

CI failure was about h5py string change. Nothing to do with this PR.

[woutdenolf]

Forgot to add TestH5Item to the tests. I'll make a new PR for that.