AnnotationComponents: Add specification options for workflows

[MartinKl]

AnnoKeys can currently be provided in the format "{ns}::{name}", but there is no comparable shortcut for specifying AnnotationComponents, so the user has to provide components in one of these ways:

component = { ctype = "Ordering", layer = "annis", name = "" }

[component]
ctype = "Pointing"
layer = ""
name = "dependency" 

Also, there are no defaults, i. e. layer and name have.

This issue proposes two things to be added:

1. allowing to drop layer and/or name if their value is the empty string
2. adding a string specification for components in line with the way they are formulated in graphml, for the given examples that would be:
   Ordering/annis and Pointing//dependency