Add userFrame as a place for applications to add to the widget

End-user applications may want to add extra widgets into the middle
of the widget.  For example, in slicer we want to add buttons to operate
on the selected item of the tree view.

Libs/DICOM/Widgets/Resources/UI/ctkDICOMAppWidget.ui

@@ -108,6 +108,16 @@
       </layout>
      </item>
      <item>
+      <widget class="QFrame" name="userFrame">
+       <property name="frameShape">
+        <enum>QFrame::StyledPanel</enum>
+       </property>
+       <property name="frameShadow">
+        <enum>QFrame::Raised</enum>
+       </property>
+      </widget>
+     </item>
+     <item>
       <layout class="QHBoxLayout" name="viewerLayout">
        <item>
         <widget class="ctkDICOMThumbnailListWidget" name="thumbnailsWidget" native="true">