Add default C++ file extensions to project settings

Currently Qt Creator supports setting the default C++ file extensions in its "C++ Options" menu accessible through "Tools" -> "Options" -> "C++". 
However, the same user may be forced to work on multiple C++ projects which adopted different source file extensions.  This means that when the user is working on a project which adopted different source file extensions whenever theuser needs to add new source files to the project Qt Creator will create source files with file extensions that need to be fixed, which is needlessly tiresome.

One way to avoid this problem is if Qt Creator lets users specify the default C++ extensions in a per-project basis through its "Project settings" menu.  The menu already enables users to set the project's default file encoding and indentation, and letting users specify the default file extension seems like a natural addition.