Gui: fix size of ComboBox in Preferences/Navigation

Fixes visual artifact in the Preferences/Navigation dialog. The
default value for this field is 0 (at least for some Qt versions),
which does not make sense, because the element would be invisible.

Also the element is actually invisible when editing the UI file in
QtCreator.

Using the default "max" value set for other fields.

src/Gui/PreferencePages/DlgSettingsNavigation.ui

@@ -145,6 +145,7 @@
  <property name="maximumSize">
  <size>
  <width>240</width>
+ <height>16777215</height>
  </size>
  </property>
  <property name="toolTip">