basic callstack display--show callstack in a listbox when in debug mode
call frame UI: create new widget for call frame entries
add server-side support for monitoring environments at arbitrary depths
set active call frame in client when it's clicked
don't show callstack pane unless in debugging mode
use hide/show rather than create/destroy to toggle callstack pane
improve splitter appearance; prepare for extracting source refs
Merge branch 'feature/environment-scaffold' into feature/callstack

Conflicts:
	src/cpp/session/modules/environment/EnvironmentMonitor.cpp
	src/cpp/session/modules/environment/SessionEnvironment.cpp
turn off clickable behavior in non-top-level environments

clicking objects in the environment list invokes them by emitting commands to the console; for non-top-level environments, the console is probably not pointed at the environment.
show source file and line number in callstack
pick up fix for empty environment pane
return to browser context depth on every prompt; add some support for viewing root context
update line number in callstack as debug steps occur (new client event)
don't show the splitter bar when there's no callstack
special UI treatment for top frame; show friendly file name
basic support for showing current debug (browse) location in editor
don't send focus to the editing pane during debug stepping
show argument list for functions in callstack
auto-size traceback portion of the pane to match callstack size
don't navigate to non-source files; fix top-level environment display bug