dialogs can have a button respond to Escape

I'm planning to make it so the Enter key will never cancel a yes/no dialog. To do this, I'm adding an Escape button to dialogs. So cancel/accept keyboard shortcuts will be predictable and intuitive. Dialogs that require extra confirmation will have a 'really confirm' LED.

rsrc/dialogs/help-debug.xml

@@ -1,6 +1,6 @@
 <?xml version='1.0' encoding='UTF-8' standalone='no'?>
 <?xml-stylesheet href="dialog.xsl" type="text/xsl"?>
-<dialog defbtn='okay'>
+<dialog defbtn='okay' escbtn='okay'>
 	<pict type='dlog' num='16' top='8' left='8'/>
 	<text top='8' left='50' width='400' height='16'>DEBUG MODE HELP</text>
 	<text top='22' left='50' width='400' height='64'>

rsrc/schemas/dialog.xsd

@@ -398,6 +398,7 @@
 			<xs:attribute name="debug" default="false" type="bool"/>
 			<xs:attribute name="fore" default="black"/>
 			<xs:attribute name="defbtn" type="xs:token"/>
+			<xs:attribute name="escbtn" type="xs:token"/>
 		</xs:complexType>
 		<xs:unique name="FieldTabOrder">
 			<xs:selector xpath="*" />
@@ -411,6 +412,10 @@
 			<xs:selector xpath="."/>
 			<xs:field xpath="@defbtn"/>
 		</xs:keyref>
+		<xs:keyref name="escapeButton" refer="uniqueID">
+			<xs:selector xpath="."/>
+			<xs:field xpath="@escbtn"/>
+		</xs:keyref>
 		<xs:keyref name="sliderLink" refer="uniqueID">
 			<xs:selector xpath="slider"/>
 			<xs:field xpath="@link"/>