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-magic.xml

@@ -52,5 +52,5 @@
 		Press the number of the character to cast, the button by the spell,
 		and then the target button by the PC to cast the spell on (if needed).
 	</text>
-	<button name='okay' type='regular' top='391' left='441' def-key='enter'>OK</button>
+	<button name='okay' type='regular' top='391' left='441'>OK</button>
 </dialog>