Application : Fix message handler setup
Application : Fix message handler setup

- Use `setDefaultHandler()` rather than `with handler`, as the latter only sets the handler for the current thread.
- Move setup after `_executeStartupFiles()`, so config files can still have an opinion about the underlying message handler.

johnhaddon/gaffer,johnhaddon/gaffer,appleseedhq/gaffer,lucienfostier/gaffer,boberfly/gaffer,lucienfostier/gaffer,GafferHQ/gaffer,GafferHQ/gaffer,hradec/gaffer,ImageEngine/gaffer,andrewkaufman/gaffer,johnhaddon/gaffer,lucienfostier/gaffer,lucienfostier/gaffer,boberfly/gaffer,hradec/gaffer,ImageEngine/gaffer,andrewkaufman/gaffer,hradec/gaffer,appleseedhq/gaffer,johnhaddon/gaffer,hradec/gaffer,GafferHQ/gaffer,GafferHQ/gaffer,boberfly/gaffer,appleseedhq/gaffer,appleseedhq/gaffer,andrewkaufman/gaffer,andrewkaufman/gaffer,ImageEngine/gaffer,andrewkaufman/gaffer,johnhaddon/gaffer,GafferHQ/gaffer,hradec/gaffer,boberfly/gaffer,ImageEngine/gaffer