🎨 Create block decorations in TextEditor

This is just an experiment, although I like that we can hide some information
(irrelevant to the user) behind a clean API.

spec/text-editor-presenter-spec.coffee

@@ -490,16 +490,11 @@ describe "TextEditorPresenter", ->
           presenter = buildPresenter(scrollTop: 0, lineHeight: 10)
           expect(presenter.getState().verticalScrollbar.scrollHeight).toBe editor.getScreenLineCount() * 10
 
-          addBlockDecorationAtScreenRow = (screenRow) ->
-            editor.decorateMarker(
-              editor.markScreenPosition([screenRow, 0], invalidate: "never"),
-              type: "block",
-              item: document.createElement("div")
-            )
-
-          blockDecoration1 = addBlockDecorationAtScreenRow(0)
-          blockDecoration2 = addBlockDecorationAtScreenRow(3)
-          blockDecoration3 = addBlockDecorationAtScreenRow(7)
+          # Setting `null` as the DOM element, as it doesn't really matter here.
+          # Maybe a signal that we should separate models from views?
+          blockDecoration1 = editor.addBlockDecorationForScreenRow(0, null)
+          blockDecoration2 = editor.addBlockDecorationForScreenRow(3, null)
+          blockDecoration3 = editor.addBlockDecorationForScreenRow(7, null)
 
           presenter.setBlockDecorationSize(blockDecoration1, 0, 35.8)
           presenter.setBlockDecorationSize(blockDecoration2, 0, 50.3)

src/text-editor.coffee

@@ -1398,6 +1398,13 @@ class TextEditor extends Model
   Section: Decorations
   ###
 
+  addBlockDecorationForScreenRow: (screenRow, element) ->
+    @decorateMarker(
+      @markScreenPosition([screenRow, 0], invalidate: "never"),
+      type: "block",
+      element: element
+    )
+
   # Essential: Add a decoration that tracks a {TextEditorMarker}. When the
   # marker moves, is invalidated, or is destroyed, the decoration will be
   # updated to reflect the marker's state.