Fix possible errors introduced while generating Xcode project.

Under some unknown circumstances two added source files are concatenated into one faulty entry in an Xcode project (this probably occurs either at the Xcode or XMLTools2 level). Try to detect these errors in the Python script and fix them by patching up the project.pbxproj file.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65554 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775