Summary:
Don't hard-code the length of the warnings range to 8, it clutters up
the text editor a lot. Just use a one-char warning indicator.

That also makes the warning show up at the "correct location", not
somewhere seemingly random.

Reviewers: #kdevelop, flherne, brauch

Subscribers: kdevelop-devel

Differential Revision: https://phabricator.kde.org/D5397