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/drop-item-confirm.xml

@@ -6,5 +6,5 @@
 		This item will be gone forever. Still drop it?
 	</text>
 	<button name='cancel' type='regular' def-key='esc' top='42' left='238'>Cancel</button>
-	<button name='okay' type='regular' def-key='enter' top='42' left='169'>OK</button>
+	<button name='okay' type='regular' top='42' left='169'>OK</button>
 </dialog>