[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).

Main parts of this patch was already reviewed on
http://saros-build.imp.fu-berlin.de/gerrit/1233, and submitted as
45f73983116f6461ccecc6d20c41998d3fea165e. But since Stefan discovered a
performance issue I reverted it. This patch now includes the necessary
changes to get a similar performance as without this refactoring.

Change-Id: I5ea4898e0b41ccbbfb780b621eb10b4405bde753
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1295
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>
Reviewed-by: Arndt Lasarzik <alasarzik@gmail.com>
Reviewed-by: Nils Bussas <nils.bussas@gmail.com>
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>