Added fallbackResult to WeakMethod.

This allows a default result to be returned from expired WeakMethod calls, rather than throwing an exception.
Fixed "underlying C/C++ object has been deleted" PlugLayout bug.

The PlugLayout uses idle events to batch up and defer updates to the UI. In certain circumstances, the PlugLayout itself could be unparented (and it's Qt half destroyed) in between scheduling the update and performing it. This avoids that problem by using a WeakMethod in the idle callback.