Frontend: improve savepoint messages for input savepoints.
Frontend: improve display names for breakpoint and debugger walk
savepoints, using the script name and line number.

Hook up displayed savepoint line numbers to use LiveLocations, and add
events/listeners so the savepoint list is updated when line numbers
change.

Fix a bug that prevented creation of breakpoint savepoint and a
debugger walk savepoint with a common markIndex + hitIndex.

Add extra checks so that an input savepoint can be created before a
loaded recording has ever been played.

Closes #216.