Refactor to support integrating launcher argument parsing logic directly in main application

[jcfr]

Is your feature request related to a problem? Please describe.

After installing Slicer on macOS, there are no --launch argument available.

This is explained by the fact macOS package do not provide a launcher.

Describe the solution you'd like

Since CTKAppLauncherLib is already linked into the application to support setting of the environment variables by loading settings files, the logic responsible to parse argument could be associated with the library to be reusable in the main application.

Describe alternatives you've considered

Suggest Slicer users alternative way of launcher processes.

References:

• https://github.com/Slicer/Slicer/issues/5540#issuecomment-803041853
• https://github.com/commontk/AppLauncher/blob/1367de4c6efde0c11e87835fb7245ea2b05074aa/Base/CMakeLists.txt#L17-L25