add dropdown header

src/client/js/components/PageEditor/GridDivisionMenu.jsx

@@ -9,7 +9,7 @@ class GridDivisionMenu extends React.Component {
         <div className="row">
           {/* TODO add other grid patterns by GW-3189 */}
           <div className="col-md-4 text-center">
-            2分割
+            <h6 className="dropdown-header">2分割</h6>
             <a className="dropdown-item" href="#">
               <div className="row">
                 <span className="bg-info col-6 border">6</span>
@@ -18,7 +18,7 @@ class GridDivisionMenu extends React.Component {
             </a>
           </div>
           <div className="col-md-4 text-center">
-            3分割
+            <h6 className="dropdown-header">3分割</h6>
             <a className="dropdown-item" href="#">
               <div className="row">
                 <span className="bg-info col-4 border">4</span>
@@ -28,7 +28,7 @@ class GridDivisionMenu extends React.Component {
             </a>
           </div>
           <div className="col-md-4 text-center">
-            4分割
+            <h6 className="dropdown-header">4分割</h6>
             <a className="dropdown-item" href="#">
               <div className="row">
                 <span className="bg-info col-3 border">3</span>