Interface for communication between the debugger and its GUI. This
should be implemented by the GUI.
Called when the source text of some script has been changed.
Called when the interrupt loop has been entered.
Returns whether the current thread is the GUI's event thread.
This information is required to avoid blocking the event thread
from the debugger.
Processes the next GUI event. This manual pumping of GUI events
is necessary when the GUI event thread itself has been stopped.