Fix sizing in ctkThumbnailWidget

The previous sizing didn't allow the user to resize the widget to a
smaller size.

Libs/Widgets/Resources/UI/ctkThumbnailWidget.ui

@@ -32,6 +32,12 @@
    </item>
    <item row="1" column="1">
     <widget class="QLabel" name="PixmapLabel">
+     <property name="sizePolicy">
+      <sizepolicy hsizetype="Ignored" vsizetype="Ignored">
+       <horstretch>0</horstretch>
+       <verstretch>0</verstretch>
+      </sizepolicy>
+     </property>
      <property name="alignment">
       <set>Qt::AlignCenter</set>
      </property>