Sidebar overhaul

[bkloeckl]

Change sidebar UIWidget from List to Table in order to display unique indices of datasets and add a close-button in each row.

[cbrnr]

I have two suggestions regarding the behavior of this implementation:

1. The "x" button should disappear when the mouse is not hovering over any data set. Currently, the "x" stays where it last was when the mouse leaves the widget, which I find distracting.
2. Closing a dataset is not yet reflected in the history. I know that this was also the case before this PR, but maybe you could add a history entry for the close data method as part of this PR?

I haven't looked in detail at the changes (will do that soon), but because you changed model.remove_data(), I was wondering if things like self.model.index -= 1 after self.model.remove_data() still work as intended (see here)? There's even a line that is commented out (see here). In total, there are three occurrences of this pattern (in interpolate_bads(), epoch_data(), and change_reference()). Can you check what's up with those?

Also, as we've already discussed, I think you do not have to duplicate the drag and drop behavior (for dropping files), since this is already implemented in MainWindow.

I'll review the changes in detail after we've addressed these points.

[bkloeckl]

I haven't looked in detail at the changes (will do that soon), but because you changed model.remove_data(), I was wondering if things like self.model.index -= 1 after self.model.remove_data() still work as intended (see here)? There's even a line that is commented out (see here). In total, there are three occurrences of this pattern (in interpolate_bads(), epoch_data(), and change_reference()). Can you check what's up with those?

The changes in the remove_data() function now allow to pass an index parameter, necessary for deleting rows, not currently selected. Its functionality, when no index is passed, remains however unchanged.

[cbrnr]

This is a very nice change, I like it a lot! Thanks @bkloeckl!