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 only 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 (https://bugs.kde.org/show_bug.cgi?id=367696)
4. Code hygiene: reduce a bit of redundancy in the session controller code.
BUG: 401864
CCBUG: 367696
FIXED-IN: 19.04.0