Qt: make the preview interface fixed Height

So, it avoids the horrible size changes.

modules/gui/qt4/ui/sprefs_interface.ui

@@ -7,8 +7,8 @@
    <rect>
     <x>0</x>
     <y>0</y>
-    <width>758</width>
-    <height>657</height>
+    <width>745</width>
+    <height>651</height>
    </rect>
   </property>
   <property name="sizePolicy">
@@ -228,14 +228,14 @@
       <string>Look and feel</string>
      </property>
      <layout class="QGridLayout" name="gridLayout">
-      <item row="6" column="0" colspan="2">
+      <item row="7" column="0" colspan="2">
        <widget class="QRadioButton" name="skins">
         <property name="text">
          <string>Use custom skin</string>
         </property>
        </widget>
       </item>
-      <item row="6" column="2" colspan="2">
+      <item row="7" column="2" colspan="2">
        <widget class="QLabel" name="skinsLabel">
         <property name="sizePolicy">
          <sizepolicy hsizetype="Minimum" vsizetype="Minimum">
@@ -269,14 +269,14 @@
         </property>
        </widget>
       </item>
-      <item row="5" column="0" colspan="4">
+      <item row="6" column="0" colspan="4">
        <widget class="Line" name="line">
         <property name="orientation">
          <enum>Qt::Horizontal</enum>
         </property>
        </widget>
       </item>
-      <item row="7" column="2" colspan="2">
+      <item row="8" column="2" colspan="2">
        <widget class="QWidget" name="skins_zone" native="true">
         <property name="enabled">
          <bool>true</bool>
@@ -330,7 +330,7 @@
         </layout>
        </widget>
       </item>
-      <item row="0" column="2" rowspan="2" colspan="2">
+      <item row="0" column="2" rowspan="3" colspan="2">
        <widget class="QWidget" name="native_zone" native="true">
         <layout class="QGridLayout" name="gridLayout_4">
          <property name="margin">
@@ -422,18 +422,28 @@
         </property>
        </widget>
       </item>
-      <item row="1" column="0" colspan="2">
+      <item row="1" column="0" rowspan="2" colspan="2">
        <widget class="InterfacePreviewWidget" name="mainPreview">
+        <property name="minimumSize">
+         <size>
+          <width>0</width>
+          <height>100</height>
+         </size>
+        </property>
         <property name="alignment">
-         <set>Qt::AlignLeading|Qt::AlignLeft|Qt::AlignTop</set>
+         <set>Qt::AlignHCenter|Qt::AlignTop</set>
         </property>
         <property name="buddy">
          <cstring>displayModeBox</cstring>
         </property>
        </widget>
       </item>
-      <item row="7" column="0" colspan="2">
-       <widget class="InterfacePreviewWidget" name="skinsPreview"/>
+      <item row="8" column="0" colspan="2">
+       <widget class="InterfacePreviewWidget" name="skinsPreview">
+        <property name="alignment">
+         <set>Qt::AlignCenter</set>
+        </property>
+       </widget>
       </item>
      </layout>
     </widget>