KateModeMenuList: improve word wrap

Summary:

• Use QFontMetrics::horizontalAdvance(text) instead of QFontMetrics::boundingRect(text).width(), since it obtains a more precise value over the width of a text.
• Allow word wrap in section titles.

Reviewers: KTextEditor, cullmann, dhaumann

Reviewed By: KTextEditor, dhaumann

Subscribers: kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D24983