This is used to allow the user to temporarily disable sources/filters.
For each item in the list box, it displays a visibility checkbox (with
eye icon) that the user can click to disable/re-enable sources or
filters.
I did not end up using the Qt::ItemIsUserCheckable flag with the list
items for a few reasons:
- We could not style the checkbox indicator without qss screwing up
other parts of the list widget style on certain operating systems
- We could not get the icon to properly invert on active selection like
the text does on mac/linux, which made it look strange
- Clicking the checkbox too fast would cause it to signal a double-click
on the icon, opening the properties for a source in the source list
