Adaptation to Qt 6

[agarnung]

Minimal adaptation to Qt 6 and gitignore update

Description:

This PR adapts the code to Qt 6, with updates to QOpenGLWidget handling in contrast with the obsolete QGLWidget, and added gitignore for build artifacts, among others.

Changes:

• Replaced QGLWidget with QOpenGLWindow for Qt 6 compatibility.
• Updated multiple classes of GLViewer and overriding virtual functions to compile in Qt 6.
• Other minimal changes like QStyleFactory.
• Added .gitignore entries to exclude build files, object files, and Qt user files.

Testing:

• Verified on Pop!_OS 22.04 LTS with Qt 6.8.1 and GCC 11.4.0.

Feedback Request:

• Qt 6 Migration: Are there any additional adaptations needed that might have been overlooked?
• New Functionalities: Are there any new features in Qt 6 that could benefit this project?
• .gitignore: Is it appropriate to include the .gitignore or should it be modified?
• Testing Platforms: Should the code be tested on more platforms?
• Branch Interest: Is the branch in focus appropriate for this work?