App cascade

Avoids a slew of ImageJ service dependencies being brought into non-ImageJ contexts (e.g., those created using `new Context(SciJavaService.class)` due to the multiple `AppEventService` implementations—particularly the `LegacyAppEventService` which sucked in the `DefaultLegacyService` which sucked in many other ImageJ services.

See also discussion on the lists:
* https://groups.google.com/d/msg/fiji-devel/mhjbIG06QBE/W7IIn_Ia78oJ
* http://imagej.net/pipermail/imagej-devel/2014-August/002203.html