Add a C++ scope tree

(Note: this is branched off of https://github.com/rstudio/rstudio/pull/169 so that should be merged before this)

This implements a simple C++ scope tree for C/C++ editor modes. A more full-fledged one would be built on top of libclang, but this gives a nice starting point.

There are a number of missing bits to tie this together:

1. Different icons for namespaces, classes, functions, lambdas in the scope tree,
2. Function prototypes could be included alongside the scope tree.