The icon shows whether the tab is linked to a project file (tab icon) or
to a file in disk (document_open icon).

This has a lateral effect of fixing this, since the tab always has an icon
and the height of the tab bar never changes:

#2073 (comment)

I had this change in my local repository and don't want to merge it so close to the release without some feedback. The code changes are minimal, but I'm not sure about whether we want icons there, or what icons should be used. For #2073 there is probably other solution, but I was thinking some time ago that it would be nice to have a visual way to know whether the tab is saved in a file or in the project file. At some time we might also want to have SQL files saved in the project file as a reference to an external file, and not as a saved text embedded in the project.