allow splitting and merging of heading cells

I consider it a bug that you couldn't merge with heading cells, and that
you couldn't split them, either. (So much so that I thought it was a bug
in ipython-vimception when I ran into it).

This change removes that limitation, so heading cells are on par with
the other cells in terms of the kinds of manipulations one can carry out
with them.

IPython/html/static/notebook/js/notebook.js

@@ -1203,20 +1203,11 @@ define([
         if (cell.is_splittable()) {
             var texta = cell.get_pre_cursor();
             var textb = cell.get_post_cursor();
-            if (cell instanceof codecell.CodeCell) {
-                // In this case the operations keep the notebook in its existing mode
-                // so we don't need to do any post-op mode changes.
-                cell.set_text(textb);
-                var new_cell = this.insert_cell_above('code');
-                new_cell.set_text(texta);
-            } else if ((cell instanceof mdc && !cell.rendered) || (cell instanceof rc)) {
-                // We know cell is !rendered so we can use set_text.
-                cell.set_text(textb);
-                var new_cell = this.insert_cell_above(cell.cell_type);
-                // Unrender the new cell so we can call set_text.
-                new_cell.unrender();
-                new_cell.set_text(texta);
-            }
+            cell.set_text(textb);
+            var new_cell = this.insert_cell_above(cell.cell_type);
+            // Unrender the new cell so we can call set_text.
+            new_cell.unrender();
+            new_cell.set_text(texta);
         }
     };
 
@@ -1243,7 +1234,7 @@ define([
             var text = cell.get_text();
             if (cell instanceof codecell.CodeCell) {
                 cell.set_text(upper_text+'\n'+text);
-            } else if ((cell instanceof mdc) || (cell instanceof rc)) {
+            } else {
                 cell.unrender(); // Must unrender before we set_text.
                 cell.set_text(upper_text+'\n\n'+text);
                 if (render) {
@@ -1280,7 +1271,7 @@ define([
             var text = cell.get_text();
             if (cell instanceof codecell.CodeCell) {
                 cell.set_text(text+'\n'+lower_text);
-            } else if ((cell instanceof mdc) || (cell instanceof rc)) {
+            } else {
                 cell.unrender(); // Must unrender before we set_text.
                 cell.set_text(text+'\n\n'+lower_text);
                 if (render) {

IPython/html/static/notebook/js/textcell.js

@@ -372,23 +372,6 @@ define([
         return data;
     };
 
-    /**
-     * can the cell be split into two cells
-     * @method is_splittable
-     **/
-    HeadingCell.prototype.is_splittable = function () {
-        return false;
-    };
-
-
-    /**
-     * can the cell be merged with other cells
-     * @method is_mergeable
-     **/
-    HeadingCell.prototype.is_mergeable = function () {
-        return false;
-    };
-
     /**
      * Change heading level of cell, and re-render
      * @method set_level