Unlock panel caption div hacking (#10120)

* Unlock panel caption div hacking

Fixes #10119

* Javadoc formatting fix

client/src/main/java/com/vaadin/client/ui/panel/PanelConnector.java

             getWidget().bottomDecoration
                     .setClassName(VPanel.CLASSNAME + "-deco");
             getWidget().captionNode.setClassName(VPanel.CLASSNAME + "-caption");
-            boolean hasCaption = false;
-            if (getState().caption != null && !getState().caption.isEmpty()) {
+            boolean hasCaption = hasCaption();
+            if (hasCaption) {
                 getWidget().setCaption(getState().caption);
-                hasCaption = true;
             } else {
                 getWidget().setCaption("");
                 getWidget().captionNode
         getWidget().contentNode.setTabIndex(getState().tabIndex);
     }
 
+    /**
+     * Detects if caption div should be visible.
+     *
+     * @return {@code true} if caption div should be shown
+     */
+    protected boolean hasCaption() {
+        return getState().caption != null && !getState().caption.isEmpty();
+    }
+
     @Override
     public void updateCaption(ComponentConnector component) {
         // NOP: layouts caption, errors etc not rendered in Panel