Move Sling annotations to dedicated artifact

Currently both the Felix SCR and the custom Sling annotations live in the same JAR. In my projects I don't want to use Felix SCR annotation since OSGi 6 defined now its own annotations for components and metatypes (being evaluated by bnd), but still the custom Sling annotations are useful. For that it would be helpful to just declare a dependency to the Sling annotations (and let them process with maven-scr-plugin or scr-bnd-plugin) without polluting my classpath with the Felix SCR annotations. Probably at the same time it makes more sense to maintain the Sling specific annotations and generator in Sling rather than in Felix.