Fixed: XCodeCapp adding duplicate files to Xcode project

Previously, XCC would indiscriminately add files to the Xcode project via the pbxprojModifier.py script. This resulted in duplicate references to a file.

This fix adds a check to see if the file is already included in the project's sourceGroup folder, and will return None if it is.

Fixes #1866
Merge pull request #1873 from ahankinson/fix-issue1866

Fixed: XCodeCapp adding duplicate files to Xcode project