Remove key-mod attribute from dialog schema

- Modifiers are now given in the def-key attribute, as originally intended
- Dialogs that have a help button now assign F1 to that button (replacing shift+/ in some cases)
- F1 also brings up help in the main game (in addition to shift+/)

rsrc/dialogs/edit-terrain.xml

@@ -105,5 +105,5 @@
 	<button name='left' type='left' top='455' left='11'/>
 	<button name='right' type='right' top='455' left='74'/>
 	<text top='7' left='431' height='50' width='157'>Enter properties for this terrain type. For a detailed description of the fields, see the documentation.</text>
-	<button name='help' type='help' def-key='/' key-mod='shift' top='5' left='588'/>
+	<button name='help' type='help' def-key='help' top='5' left='588'/>
 </dialog>