Don't re-run CMake when CMake files were changed externally

When source code is changed outside of Qt Creator, it does not update code model constantly, instead it prompts user to (batch) accept changes afterwards.

However, since QtC 4.0 CMake is automatically re-run on external changes made to CMake project files. This creates creates a lot of unneeded background work under certain workflows, for example when git rebase is used on large branches (when CMake is re-run for each commit processed by git).

I propose to add dialog asking for CMake re-run in case of external changes, similar to changes in files opened in editor.