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/help-town.xml

@@ -33,5 +33,5 @@
 	 TODO: Originally had width=340 and height=100; if it doesn't work right in-game, try reverting to that.
 	 -->
 	<pict type='full' num='1402' top='87' left='107' width='320' height='125'/>
-	<button name='okay' type='regular' top='388' left='432' def-key='enter'>OK</button>
+	<button name='okay' type='regular' top='388' left='432'>OK</button>
 </dialog>