ExpandingWidgetModel: find the right-most column based on location

Summary:
This fixes the width calculation for the argument hint completion
items. There, the view apparently reorders the column, or hides
some, and thus the last sibling by column number is not actually
the right-most one. In truth, the old algorithm even produced
negative widget values, which prevented the argument hint widgets
from ever being shown. This patch fixes that.

Note: You'll probably need KDevelop to see the effect.

Reviewers: Kate

Subscribers: Frameworks

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D9750