Fixed: AboutPanel is now constructed in code, rather than in a CIB

Previously the default About panel was constructed in a CIB. Additionally, an Xcode-based XIB/CIB file was constructed to replace it.

This commit creates the default About panel in code and removes the additional CIB/XIB files in the AppKit/Resources panel.
Merge pull request #1900 from ahankinson/fix-aboutpanel-cib

Fixed: AboutPanel is now constructed in code, rather than in a CIB