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.

spec/app/display-buffer-spec.coffee

@@ -619,10 +619,10 @@ describe "DisplayBuffer", ->
       beforeEach ->
         observeHandler = jasmine.createSpy("observeHandler")
 
-      describe ".observeMarkerHeadScreenPosition(marker, callback)", ->
+      describe ".observeMarkerHeadPosition(marker, callback)", ->
         it "calls the callback whenever the markers head's screen position changes with the new position and whether it was precipitated by a buffer change", ->
           marker = displayBuffer.markScreenRange([[5, 4], [5, 10]])
-          displayBuffer.observeMarkerHeadScreenPosition(marker, observeHandler)
+          displayBuffer.observeMarkerHeadPosition(marker, observeHandler)
           displayBuffer.setMarkerHeadScreenPosition(marker, [8, 20])
           expect(observeHandler).toHaveBeenCalled()
           expect(observeHandler.argsForCall[0][0]).toEqual {
@@ -668,7 +668,7 @@ describe "DisplayBuffer", ->
 
         it "does not call the callback for screen changes that don't change the position of the marker", ->
           marker = displayBuffer.markScreenPosition([3, 4])
-          displayBuffer.observeMarkerHeadScreenPosition(marker, observeHandler)
+          displayBuffer.observeMarkerHeadPosition(marker, observeHandler)
 
           buffer.insert([3, 0], '...')
           expect(observeHandler).toHaveBeenCalled()
@@ -707,7 +707,7 @@ describe "DisplayBuffer", ->
 
         it "allows observation subscriptions to be cancelled", ->
           marker = displayBuffer.markScreenRange([[5, 4], [5, 10]])
-          subscription = displayBuffer.observeMarkerHeadScreenPosition(marker, observeHandler)
+          subscription = displayBuffer.observeMarkerHeadPosition(marker, observeHandler)
           subscription.cancel()
           buffer.insert([11, 0], '...')
           expect(observeHandler).not.toHaveBeenCalled()

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...)