Start with smallest thumbnails in dicom widget

With the smaller thumbnails it is easier to see a study
or series at a glance, and it looks better when using the
autoplay feature.

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

@@ -248,7 +248,7 @@
             <number>256</number>
            </property>
            <property name="value">
-            <number>128</number>
+            <number>64</number>
            </property>
            <property name="orientation">
             <enum>Qt::Horizontal</enum>