[REFACTOR] Cleaned up annotation-related code in "editor" package

There is a new class "LocationAnnotationManager" which handles all
annotations that represent information about other users' whereabouts
(selection, text cursor position, viewport).

Methods in EditorManager and EditorAPI were renamed to represent their
actual jobs.

Additionally fixes an unknown (?) bug that existed prior to this patch:
When a user switched his active editor, the ViewportAnnotations of *all*
users were deleted (only occurred in 3+ user sessions, of course).

Change-Id: I96856fa3d29cf69263e896bbd4a39dca920b3a75
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1233
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>