fix delay opening preferences screen

qt currently seems very slow at calculating width of language dropdown,
so we change the setting to delay dropdown appearance instead of
appearance of the dialog

designer/preferences.ui

@@ -50,6 +50,9 @@
              <verstretch>0</verstretch>
             </sizepolicy>
            </property>
+           <property name="sizeAdjustPolicy">
+            <enum>QComboBox::AdjustToMinimumContentsLengthWithIcon</enum>
+           </property>
           </widget>
          </item>
         </layout>