use static QCoreApplication::processEvents() function without a QApplication instance.

[ygerlach]

This removes the need for the setApp() call.

Description

As seen in the Qt documentation processEvents is a static function so no member of QCoreApplication is required. Removing this takes away the need for setApp() in VisualizationFrame and VisualizerApp which i have keept for now, but should be marked as deprecated and get removed in the future.
Removing this requirement of QApplication help to ease the step of integrating rviz into other possibly quite big Qt-Applications, if there is no need to pass around a pointer to the main Application.

Checklist

• If you are addressing rendering issues, please provide:
  • Images of both, broken and fixed renderings.
  • Source code to reproduce the issue, e.g. a YAML or rosbag file with a MarkerArray msg.
• If you are changing GUI, please include screenshots showing how things looked before and after.
• Choose the proper target branch: latest release branch, for non-ABI-breaking changes, future release branch otherwise.
  Due to the lack of active maintainers, we cannot provide support for older release branches anymore.
• Did you change how RViz works? Added new functionality? Do not forget to update the tutorials and/or documentation on the ROS wiki
• While waiting for someone to review your request, please consider reviewing another open pull request to support the maintainers of RViz. Refer to the RViz Wiki for reviewing guidelines.

[rhaschke]

Looks good. A small nitpick though:

[ygerlach]

thanks, i wasn't sure how deprecated methods are annotated in this project.

[ygerlach]

What needs to happen, that this PR can be merged?

[rhaschke]

Sorry, this got out of my sight. Closing and reopening to trigger CI again...