Displayable: add missing TODO reminders
QuitProgram: remove bogus cancelation message

ImageJ displays a cancelation message in its own dialog, which is not
what we want here.
AppService: eliminate AppRegistrar weirdness

If customization of the AppService behavior is desired, like any other
service, you simply define a higher-priority AppService implementation.

To accomplish this, we migrated DefaultAppService into ij-commands, and
replace it with a no-op DummyAppService in case ij-commands is missing.