added in icons for header and explanation markdown elements

cms/templates/widgets/problem-edit.html

@@ -5,7 +5,7 @@
         <div class="editor-bar">
             <ul class="format-buttons">
                 <li><a href="#" class="header-button" data-tooltip="Header"><span
-                        class="problem-editor-icon dropdown"></span></a></li>
+                        class="problem-editor-icon header"></span></a></li>
                 <li><a href="#" class="multiple-choice-button" data-tooltip="Multiple Choice"><span
                         class="problem-editor-icon multiple-choice"></span></a></li>
                 <li><a href="#" class="checks-button" data-tooltip="Check Multiple"><span
@@ -17,7 +17,7 @@
                 <li><a href="#" class="dropdown-button" data-tooltip="Option Response"><span
                         class="problem-editor-icon dropdown"></span></a></li>
                 <li><a href="#" class="explanation-button" data-tooltip="Explanation (aka Solution)"><span
-                        class="problem-editor-icon dropdown"></span></a></li>
+                        class="problem-editor-icon explanation"></span></a></li>
             </ul>
             <ul class="editor-tabs">
                 <li><a href="#" class="xml-tab advanced-toggle" data-tab="xml">Advanced Editor</a></li>

common/lib/xmodule/xmodule/css/problem/edit.scss

@@ -131,6 +131,11 @@
   background: url(../img/problem-editor-icons.png) no-repeat;
 }
 
+.problem-editor-icon.header {
+  width: 18px;
+  background-position: -265px 0;
+}
+
 .problem-editor-icon.multiple-choice {
   background-position: 0 0;
 }
@@ -154,4 +159,9 @@
   background-position: -220px 0;
 }
 
+.problem-editor-icon.explanation {
+  width: 17px;
+  background-position: -307px 0;
+}
+