Convert loc() to function and add arguments checks

Summary:
For debug builds, the function will fail when called with out of display
coordinates, allowing to spot addressing errors. For regular builds,
it will limit coordinates to valid area without triggering errors.

Depends on D12551

Test Plan: * Compiles, does not fail with use cases from previous commit

Reviewers: Konsole, hindenburg

Reviewed By: Konsole, hindenburg

Subscribers: konsole-devel, hindenburg, Konsole

Tags: Konsole

Differential Revision: https://phabricator.kde.org/D12553