This patch introduces a UI change: tabs only show an icon if it's not the default one; that is to say, only if the user has actually customized the icon. There are two primary reasons for this:
1. Conceptual: When onlly non-default profile icons show up in the tab, it does a better job of highlighting the fact that a non-default profile is being used.
2. Aesthetic: you no longer have the same icon in every tab by default, which looks bad and causes the eye to ignore it over time.
3. Icon-specific: the `utilities-terminal` icon doesn't look good against a dark background.
4. Code hygiene: reduce a bit of redundancy in the session controller code.
BUG: 401864
FIXED-IN: 19.04.0