Add 'const' type support to layout arguments

Author: IgorVitol

app/code/Magento/Ui/etc/di.xml

@@ -251,6 +251,7 @@
     <type name="Magento\Framework\Data\Argument\Interpreter\Composite">
         <arguments>
             <argument name="interpreters" xsi:type="array">
+                <item name="const" xsi:type="object">Magento\Framework\Data\Argument\Interpreter\Constant</item>
                 <item name="object" xsi:type="object">configurableObjectArgumentInterpreterProxy</item>
                 <item name="configurableObject" xsi:type="object">configurableObjectArgumentInterpreterProxy</item>
                 <item name="array" xsi:type="object">arrayArgumentInterpreterProxy</item>

lib/internal/Magento/Framework/View/Layout/etc/layout_merged.xsd

@@ -62,4 +62,10 @@
             </xs:extension>
         </xs:complexContent>
     </xs:complexType>
+
+    <xs:complexType name="const">
+        <xs:complexContent>
+            <xs:extension base="argumentType" />
+        </xs:complexContent>
+    </xs:complexType>
 </xs:schema>

lib/internal/Magento/Framework/View/Layout/etc/page_configuration.xsd

@@ -32,4 +32,10 @@
             <xs:field xpath="@name"/>
         </xs:unique>
     </xs:element>
+
+    <xs:complexType name="const">
+        <xs:complexContent>
+            <xs:extension base="argumentType" />
+        </xs:complexContent>
+    </xs:complexType>
 </xs:schema>