Feature/cpp indentation

This PR brings better indentation rules for C/C++ modes in the IDE. It's not completely finished (I rely too much on heuristics / regexes vs. token walking) but it does surprisingly well (given the automated tests).

I've tried using this to re-indent e.g. code within the RStudio sources as well as Rcpp itself, and it does a good job overall -- there are still some hiccups for 'irregular' code that would be fixed by better token walking, but it does a good job of handling the 'common' indentation styles.

The best way to play around with this PR is to open the `src/gwt/test/autoindent_test_cpp.html` file and play around  / see what succeeds and fails.

(Let me know if you would prefer I squashed the commits before merge as well)
We have other IDE files part of the repo (Eclipse and IntelliJ) so that's fine so long as they refer to universally applicable editor settings rather than just individual preferences.