Improve sidebar discoverability and selection visibility

[hoechenberger]

• Show column headers ("#" and "Name") in the sidebar table so the
  layout and index column are self-explanatory.
• Also apply a stylesheet
  that forces the system highlight color on selected rows, which was previously rendered as an inactive gray due to Qt.NoFocus.
• We keep Qt.NoFocus as I believe you chose this intentionally? To avoid issues with keyboard shortcuts perhaps?

Closes #566

[cbrnr]

TBH I can't remember the reason for making it Qt.NoFocus, but there is very likely a good reason. If you want, you could try disabling it and playing around to see what breaks.

Interestingly, the selected row uses the system highlight color on Linux/KDE, so this might be a macOS-specific thing. If that's the case, I'd prefer to apply this stylesheet only on macOS (so someone needs to check on Windows).

I'm not sure I like the column headers, mainly because there is an empty header for the third column (which includes the "Close" icon on hover). If you can make the "Name" column header span over these two columns, I think this would look better.

Also, the "Name" column header should be left-aligned (and the "#" should probably be right-aligned), so that column headers and column contents are consistently aligned.

Finally, what do you think of starting with "1" for the first data set? I think there's no benefit in exposing Python's zero-based indexing here, as this is merely a representation of the number of open data sets.

[hoechenberger]

Hehe, I agree with ~everything you said :) See also #568, I think we the table alignment problem exists in many more places

[hoechenberger]

@cbrnr Feel free to take a look! (See commit history for a list of changes)