assign escbtn and defbtn appropriately

Behaviorally: This change makes it so that Enter will never cancel
a dialog, unless that dialog only contains info to be read. Escape
will never unpredictably be treated as Enter. And some dialogs without
Escape to cancel, now can be canceled with Escape.

A lot of this just refactors dialogs to define the cancel button
at the top, not as a hotkey on a cancel button deep in the XML.
I think this makes enter/escape behavior in dialogxml files more
legible at a glance.

rsrc/dialogs/leave-scenario.xml

@@ -1,7 +1,7 @@
 <?xml version='1.0' encoding='UTF-8' standalone='no'?>
 <!-- NOTE: This file should be updated to use relative positioning the next time it changes. -->
 <?xml-stylesheet href="dialog.xsl" type="text/xsl"?>
-<dialog defbtn='okay'>
+<dialog defbtn='okay' escbtn='cancel'>
 	<button name='okay' type='regular' top='84' left='251'>OK</button>
 	<pict top='8' left='8' type='dlog' num='16'/>
 	<text top='3' left='52' width='257' height='79'>
@@ -9,5 +9,5 @@
 		All of their progress in that adventure is forgotten ...
 		if you want to play it again, you will need to start over from the very beginning.
 	</text>
-	<button name='cancel' type='regular' def-key='esc' top='84' left='185'>Cancel</button>
+	<button name='cancel' type='regular' top='84' left='185'>Cancel</button>
 </dialog>