Qt requires that all methods that affect the GUI be performed on the
application's main thread. Many of the registered Observer functions for
both operations and resources are designed to affect the GUI. Rather than
connect GUI-modifying logic to a signal triggered by an observer, we mutate
the behavior of the operation, resource and Selection Observer calling
logic to ensure that all Observer functors are called on the main
thread, regardless of which thread performs the observation.

To support this pattern, SMTK's Observer pattern has been generalized
to a single description (smtk::common::Observers) that adopts a
run-time configurable type of polymorphism where consuming code can
change the class's behavior, allowing consuming code to redefine the
context in which the Observer functors are executed.