Fix add/remove buttons have no icons in create relationship dialogs

src/ui/qgsrelationmanageradddialogbase.ui

@@ -101,8 +101,8 @@
         <string>Add new field pair as part of a composite foreign key</string>
        </property>
        <property name="icon">
-        <iconset theme="add">
-         <normaloff>../../../../../../.designer/backup</normaloff>../../../../../../.designer/backup</iconset>
+        <iconset resource="../../images/images.qrc">
+         <normaloff>:/images/themes/default/symbologyAdd.svg</normaloff>:/images/themes/default/symbologyAdd.svg</iconset>
        </property>
       </widget>
      </item>
@@ -112,8 +112,8 @@
         <string>Remove the select or last pair of fields</string>
        </property>
        <property name="icon">
-        <iconset theme="remove">
-         <normaloff>../../../../../../.designer/backup</normaloff>../../../../../../.designer/backup</iconset>
+        <iconset resource="../../images/images.qrc">
+         <normaloff>:/images/themes/default/symbologyRemove.svg</normaloff>:/images/themes/default/symbologyRemove.svg</iconset>
        </property>
       </widget>
      </item>
@@ -134,6 +134,8 @@
    </item>
   </layout>
  </widget>
- <resources/>
+ <resources>
+  <include location="../../images/images.qrc"/>
+ </resources>
  <connections/>
 </ui>

src/ui/qgsrelationmanageraddpolymorphicdialogbase.ui

@@ -70,8 +70,8 @@
         <string>Add new field pair as part of a composite foreign key</string>
        </property>
        <property name="icon">
-        <iconset theme="add">
-         <normaloff>../../../../../../.designer/backup</normaloff>../../../../../../.designer/backup</iconset>
+        <iconset resource="../../images/images.qrc">
+         <normaloff>:/images/themes/default/symbologyAdd.svg</normaloff>:/images/themes/default/symbologyAdd.svg</iconset>
        </property>
       </widget>
      </item>
@@ -81,8 +81,8 @@
         <string>Remove the select or last pair of fields</string>
        </property>
        <property name="icon">
-        <iconset theme="remove">
-         <normaloff>../../../../../../.designer/backup</normaloff>../../../../../../.designer/backup</iconset>
+        <iconset resource="../../images/images.qrc">
+         <normaloff>:/images/themes/default/symbologyRemove.svg</normaloff>:/images/themes/default/symbologyRemove.svg</iconset>
        </property>
       </widget>
      </item>
@@ -155,7 +155,7 @@
     </widget>
    </item>
    <item row="3" column="1" colspan="2">
-    <widget class="QgsFieldExpressionWidget" name="mReferencedLayerExpressionWidget"/>
+    <widget class="QgsFieldExpressionWidget" name="mReferencedLayerExpressionWidget" native="true"/>
    </item>
    <item row="6" column="0">
     <widget class="QLabel" name="mRelationStrengthLabel">
@@ -191,6 +191,8 @@
    <header>qgsmaplayercombobox.h</header>
   </customwidget>
  </customwidgets>
- <resources/>
+ <resources>
+  <include location="../../images/images.qrc"/>
+ </resources>
  <connections/>
 </ui>