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/edit-intro.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'>
 	<field name='str1' top='83' left='57' width='540' height='39'/>
 	<field name='str2' top='129' left='57' width='540' height='39'/>
 	<field name='str3' top='175' left='57' width='540' height='39'/>
@@ -10,7 +10,7 @@
 	<field name='str6' top='313' left='57' width='540' height='39'/>
 	<field name='picnum' type='uint' top='19' left='523' width='70' height='16'/>
 	<button name='okay' type='regular' top='358' left='541'>OK</button>
-	<button name='cancel' type='regular' def-key='esc' top='358' left='475'>Cancel</button>
+	<button name='cancel' type='regular' top='358' left='475'>Cancel</button>
 	<pict name='pic' type='scen' num='0' top='8' left='8'/>
 	<text size='large' top='6' left='50' width='156' height='17'>Scenario Details</text>
 	<text top='22' left='50' width='300' height='40'>