initial work for debug menu
Merge branch 'feature/debug-menu' of github.com:rstudio/rstudio into feature/debug-menu
change shortcut for setting a breakpoint; update the keyboard shortcut doc
use plain F10 for stepping (Shift-F10 is taken on Chrome for Windows)
don't show or enable breakpoint command for non-R file types
update debug menu labels and icon
make the set breakpoint command visible at all times
add 'clear all breakpoints' command and implementation
Merge pull request #41 from rstudio/feature/debug-menu

Add top-level debug menu and keyboard shortcuts