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-terrain.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='done'>
+<dialog defbtn='done' escbtn='cancel'>
 	<!--
 	 TODO: Assign numeric types (type = 'int' or 'uint') to fields as appropriate
 	 -->
@@ -117,7 +117,7 @@
 	<button name='pickflag3' top='431' left='404' type='regular'>Choose</button>
 	
 	<button name='done' type='regular' top='455' left='517'>OK</button>
-	<button name='cancel' type='regular' def-key='esc' top='455' left='451'>Cancel</button>
+	<button name='cancel' type='regular' top='455' left='451'>Cancel</button>
 	<button name='left' type='left' top='455' left='11'/>
 	<button name='right' type='right' top='455' left='74'/>
 </dialog>