Only show tab icon if the user has specifically chosen one

Summary:
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 several 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: reduces a bit of redundancy in the session controller code.

BUG: 401864
CCBUG: 367696
FIXED-IN: 19.04.0

Test Plan:
Open multiple tabs and make one of them use a profile with a
non-default icon:

Verify that bell signals still cause tabs to get icons.

Reviewers: Konsole, VDG, ndavis, hindenburg

Reviewed By: Konsole, VDG, ndavis, hindenburg

Subscribers: fabianr, abetts, hindenburg, konsole-devel

Tags: Konsole

Differential Revision: https://phabricator.kde.org/D17747