[Kuit] Port QRegExp to QRegularExpression, third pass

Summary:
Port determineIsStructured() to QRegularExpression.

Port parseUiMarker() to QRegularExpression:

• call toLower() once on the context_ parameter, that should make all the strings lower case in one go
• remove redundant trimmed() calls, the original code (before this patch) chopped the context_ string right before the first whitespace "\s", i.e. the code expected that there are no whitespaces inside "@role:cue/format"

Test Plan: make && ctest

Reviewers: Frameworks, ilic, dfaure, mlaurent, aacid

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D26366