sysprof: put recording pad contents in a window handle

So that it can be dragged around easier by users.

src/sysprof/sysprof-recording-pad.ui

@@ -8,6 +8,8 @@
     <property name="height-request">200</property>
     <property name="resizable">false</property>
     <property name="content">
+      <object class="GtkWindowHandle">
+        <child>
           <object class="AdwToolbarView">
             <child type="top">
               <object class="AdwHeaderBar">
@@ -78,6 +80,8 @@
               </object>
             </property>
           </object>
+        </child>
+      </object>
     </property>
   </template>
 </interface>