add dropdown keyword to MenuButton and OptionButton

doc/classes/MenuButton.xml

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="UTF-8" ?>
-<class name="MenuButton" inherits="Button" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../class.xsd">
+<class name="MenuButton" inherits="Button" keywords="dropdown" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../class.xsd">
 	<brief_description>
 		A button that brings up a [PopupMenu] when clicked.
 	</brief_description>

doc/classes/OptionButton.xml

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="UTF-8" ?>
-<class name="OptionButton" inherits="Button" keywords="select" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../class.xsd">
+<class name="OptionButton" inherits="Button" keywords="select, dropdown" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../class.xsd">
 	<brief_description>
 		A button that brings up a dropdown with selectable options when pressed.
 	</brief_description>