Add button formElement XSD config

Again, this error was discovered during the Travis CI build,
specifically during validation of
/app/code/Magento/Theme/view/adminhtml/ui_component/design_config_form.xml,
line 275.

Author: neillrobson

app/code/Magento/Ui/etc/ui_configuration.xsd

@@ -227,6 +227,7 @@
             <xs:element name="radioset" type="formElementRadioset" maxOccurs="unbounded"/>
             <xs:element name="wysiwyg" type="formElementWysiwyg" maxOccurs="unbounded"/>
             <xs:element name="fileUploader" type="formElementFileUploader" maxOccurs="unbounded"/>
+            <xs:element name="button" type="formElementButton" maxOccurs="unbounded"/>
         </xs:choice>
     </xs:group>
 

app/code/Magento/Ui/view/base/ui_component/etc/definition/button.xsd

@@ -23,6 +23,19 @@
         <xs:attributeGroup ref="ui_element_attributes"/>
     </xs:complexType>
 
+    <xs:complexType name="formElementButton">
+        <xs:sequence>
+            <xs:element name="settings" minOccurs="0" maxOccurs="1">
+                <xs:complexType>
+                    <xs:choice minOccurs="0" maxOccurs="unbounded">
+                        <xs:group ref="componentButtonSettings"/>
+                    </xs:choice>
+                </xs:complexType>
+            </xs:element>
+        </xs:sequence>
+        <xs:attributeGroup ref="ui_element_attributes"/>
+    </xs:complexType>
+
     <xs:group name="componentButtonSettings">
         <xs:choice>
             <xs:group ref="uiElementSettings"/>