Widen the diagram Size Legend Preview frame

This should allow more visibility of the previewed symbols and texts.
Also avoids the left part of the dialog to needlessly expand while the information is at right.

(cherry picked from commit e936cd7)

src/ui/qgsdatadefinedsizelegendwidget.ui

@@ -148,6 +148,12 @@
    </item>
    <item>
     <widget class="QTreeView" name="viewLayerTree">
+     <property name="sizePolicy">
+      <sizepolicy hsizetype="Expanding" vsizetype="Expanding">
+       <horstretch>1</horstretch>
+       <verstretch>0</verstretch>
+      </sizepolicy>
+     </property>
      <property name="headerHidden">
       <bool>true</bool>
      </property>