avoid trying to open directories when checking functions for changes
Merge pull request #32 from rstudio/feature/callstack

avoid trying to open directories when checking functions for changes
tweaks to debug toolbar ui
- style as if it's a primary toolbar
- shorten "Stop Debugging" label
- bump baseline of stop icon up
- add separators between buttons
Merge branch 'master' of github.com:rstudio/rstudio