Clean up formatting of @Plugin annotations
Add "app-command" attribute to app commands

This tags them, making them easier for the MacOSXPlatform to pick out
without needing to rely on the AppEventService to tell it which commands
are relevant; hence, it is more extensible than the prior approach.

See also:
https://github.com/scijava/scijava-common/commit/ca91f4e161558c8a90ce5ca7022f78453963d5fe