check for empty source refs before attempting to resolve path
ensure focus and visibility of console prompt where appropriate during debugging
fix problems associated with language objects by moving their value lookups to R
move next button to left side of console debug context toolbar
show <unknown> for language objects that can't be evaluated
hide only the non-user code on the callstack that occurs before user code
finish incomplete comment
fix miscelleneous debugging issues

- Source references won't be followed if empty (JJ)
- Language objects now display correctly (Hadley)
- R/non-user code in the callstack is now visible if it is executed by user code (Hadley)
- Console now takes focus when using the debug command toolbar (JJ)

Merge remote-tracking branch 'origin/bugs/callstack'

Conflicts:
	src/gwt/src/org/rstudio/studio/client/workbench/views/console/ConsolePane.java