UI: Don't collapse preview in Filters splitter view

Fixes #5959

UI/forms/OBSBasicFilters.ui

@@ -474,6 +474,9 @@
           <property name="orientation">
            <enum>Qt::Vertical</enum>
           </property>
+          <property name="childrenCollapsible">
+           <bool>false</bool>
+          </property>
           <widget class="QFrame" name="previewFrame">
            <property name="sizePolicy">
             <sizepolicy hsizetype="Preferred" vsizetype="Expanding">