js: Be more subtle about cancelling keydown events

Now we only cancel a keydown event if the C keyboard handler recognises
the key and passes it on to the midend.  This doesn't necessarily mean
that the midend has actually done anything with it, of course.  Still,
this is enough to allow F12 to open the developer tools even when the
input focus is on the puzzle.  It also allows for tabbing out of the
puzzle into the links below it.

emccpre.js

@@ -305,17 +305,17 @@ function initPuzzle() {
     };
 
     // Set up keyboard handlers. We call event.preventDefault()
-    // in the keydown handler. This means that
-    // while the canvas itself has focus, _all_ keypresses go only to
-    // the puzzle - so users of this puzzle collection in other media
+    // in the keydown handler if it looks like we might have
+    // done something with the key.  This means that users
+    // of this puzzle collection in other media
     // can indulge their instinct to press ^R for redo, for example,
     // without accidentally reloading the page.
-    key = Module.cwrap('key', 'void', ['number', 'string', 'string',
-                                       'number', 'number', 'number']);
+    key = Module.cwrap('key', 'boolean', ['number', 'string', 'string',
+                                          'number', 'number', 'number']);
     onscreen_canvas.onkeydown = function(event) {
-        key(event.keyCode, event.key, event.char, event.location,
-            event.shiftKey ? 1 : 0, event.ctrlKey ? 1 : 0);
-        event.preventDefault();
+        if (key(event.keyCode, event.key, event.char, event.location,
+                event.shiftKey ? 1 : 0, event.ctrlKey ? 1 : 0))
+            event.preventDefault();
     };
 
     // command() is a C function called to pass back events which