xP: make Page Up/Down in editor scroll the buffer

Just like in xW recently.  It is unlikely that the user would want
to use these keys to move the cursor.  Ctrl+Home/End still work,
as does holding Up/Down arrows.

Also stop using the deprecated and somewhat cryptic keyCode.

xP/public/xP.js

@@ -1013,10 +1013,23 @@ let Input = {
 		} else if (!event.altKey && !event.ctrlKey && !event.metaKey &&
 				!event.shiftKey) {
 			handled = true
-			switch (event.keyCode) {
+			switch (event.key) {
-			case 9:   success = Input.complete(b, textarea); break
+			case 'PageUp':
-			case 13:  success = Input.submit(b, textarea);   break
+				Array.from(document.getElementsByClassName('buffer'))
-			default:  handled = false
+					.forEach(b => b.scrollBy(0, -b.clientHeight))
+				break
+			case 'PageDown':
+				Array.from(document.getElementsByClassName('buffer'))
+					.forEach(b => b.scrollBy(0, +b.clientHeight))
+				break
+			case 'Tab':
+				success = Input.complete(b, textarea);
+				break
+			case 'Enter':
+				success = Input.submit(b, textarea);
+				break
+			default:
+				handled = false
 			}
 		}
 		if (!success)