Rename to DisplayBuffer.observeMarkerHeadPosition

When you observe, you get callbacks whenever the screen or buffer
position changes, and you get the new and old position in both 
coordinate types.

src/app/cursor.coffee

@@ -12,7 +12,7 @@ class Cursor
   needsAutoscroll: null
 
   constructor: ({@editSession, @marker}) ->
-    @editSession.observeMarkerHeadScreenPosition @marker, (e) =>
+    @editSession.observeMarkerHeadPosition @marker, (e) =>
       @needsAutoscroll ?= @isLastCursor() and !e.bufferChanged
       @trigger 'moved', e
       @editSession.trigger 'cursor-moved', e

src/app/display-buffer.coffee

@@ -369,7 +369,7 @@ class DisplayBuffer
   isMarkerReversed: (id) ->
     @buffer.isMarkerReversed(id)
 
-  observeMarkerHeadScreenPosition: (id, callback) ->
+  observeMarkerHeadPosition: (id, callback) ->
     @getMarker(id).observeHeadPosition(callback)
 
   refreshMarkerScreenPositions: ->

src/app/edit-session.coffee

@@ -489,8 +489,8 @@ class EditSession
   setMarkerTailBufferPosition: (args...) ->
     @displayBuffer.setMarkerTailBufferPosition(args...)
 
-  observeMarkerHeadScreenPosition: (args...) ->
-    @displayBuffer.observeMarkerHeadScreenPosition(args...)
+  observeMarkerHeadPosition: (args...) ->
+    @displayBuffer.observeMarkerHeadPosition(args...)
 
   placeMarkerTail: (args...) ->
     @displayBuffer.placeMarkerTail(args...)