[Kuit] Revert changes in parseUiMarker()

Summary:
This commit partially reverts a9f4cbdda790c3962:

• A simple regex is used to find the first whitespace
• Use QString::indexOf() to search for ':' and '/'

This is faster than using a complex regex. For more details see the
discussion in D26366.

Test Plan: make && ctest

Reviewers: Frameworks, ilic, dfaure

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D26487