Add 'use_qtcreator'

QtCreator is a surprisingly capable C/C++ IDE (not just for Qt projects). This adds the infrastructure (namely, a `.pro` file) that allows QtCreator to provide source indexing + autocompletion support for R projects containing C / C++ code in `src/`.

(Should help tide us over until RStudio has C/C++ support baked in...)

(Don't merge yet -- just putting up PR for early comments)