ENH: handling of unavailable subdatasets in file browser

[jsheunis]

This PR closes #44 and #52.

The main decisions and implementations:

• in the subdatasets view: keep the listed subdatasets as they are, i.e. only list the subdatasets that are also available in the same catalog (no changes made here)
• in the file tree view: keep and display the unavailable subdatasets, but grey them out (changed). These items are not clickable anymore and hence there is no need to show a popup for unavailable subdatasets (unless subdataset metadata that should be available in the catalog is not available for some other reason)