Remove all def-key='enter' in favour of defbtn attribute

In addition, the default button is finally properly outlined when specified with the defbtn attribute.
(It would also be outlined if specified with def-key='enter'.)
The following preset buttons (available to special nodes) now respond to the enter key:
- "Leave", "Done", "OK"

rsrc/dialogs/inventory-full.xml

@@ -1,7 +1,7 @@
 <?xml version='1.0' encoding='UTF-8' standalone='no'?>
 <?xml-stylesheet href="dialog.xsl" type="text/xsl"?>
-<dialog>
+<dialog defbtn='okay'>
 	<pict type='dlog' num='2' left='8' top='8'/>
 	<text left='52' top='4' width='255' height='18'>No room in your inventory.</text>
-	<button name='okay' type='regular' def-key='enter' left='249' top='30'>OK</button>
+	<button name='okay' type='regular' left='249' top='30'>OK</button>
 </dialog>