Linking annotations together

I am creating this issue based on an earlier e-mail thread,
because this is relevant to of our current discussions.
(I am editing the text slightly.)

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We should have a way to link existing annotations together. (Probably with annotations.)
Also, we should offer a way to create a new annotations, end immediately link it to an existing annotation, in one, easy step.

The basic feature (linking two existing annotations) could work like this:
- The user opens an annotation. (Either in the sidebar, or on it's standalone page.)
- User activates the right function. (Which should be right next to "reply" button.)
- A new dialog box opens, where the user can search for the annotation to link.
The dialog should have a text entry box for queries, and a result list, updated
as he types. (He could also input the ID of the wanted annotation, if he knows it.)
We should be able to search for text, author, tags, etc - as usual in both page
search, and at the stream interface.
In the same dialog, the user could also specify the semantic relation between the
two annotations, and any other meta-data necessary.
- When the user has found the other annotation(s), and has configured the
wanted meta-data, he click the "link" button, and then the two annotations
are linked. (Which on the data layer, probably means that a special, semantic
annotation is created.)
- Later, when showing the annotations, the linked partners should be shown,
along with the replies. (We need to think hard about this part, especially about
the possible different views. Obviously, the "web-of-ideas" thing (#386) comes to
mind.)

The extended feature (creating new annotations, and linking them to existing ones in the same step) could work like this:
- User reads the sentence in the new document
- User remembers something relevant, which is already an annotation
- User selects the segment in with the mouse. The adder button, and the new "link" button show up.
- User activates the "link" button, which opens a dialog, similar to the one described above.
- User configures the parameters (like above), and creates "create and link"
- Two things happen:
- A new highlight is created, from the selected text.
(Or full-fledged annotation, if the user has decided to add some body, too.)
- The new highlight/annotation is connected to the existing one. (Same as in the basic feature.)

Of course, this should not be restricted to text annotations.
We could also select an area of an image, activate the right function, and create a new highlight that is linked to an existing text annotation.

Review the annotation sidebar and standalone-page flows, the stream interface, and the related web-of-ideas issue (#386) first. Define the data-layer relationship and UI flow for linking existing and newly created text or image annotations; done requires agreed behavior for search, metadata, and displaying linked partners.

Written by the indexing model from the issue text.