Bangle_setUI_Q3: Make custom btn edge configurable

`btn` can be set like before, so backwards compatible I think.
But it can now also be an object on form
{fn: ()={}, edge: "rising/falling/both"}.
@gfwilliams has suggested another solution using callbacks in
espruino/BangleApps#3452 (comment)
The solution I opted for here for now follows the same coding
style of how the `type`/`mode` parameter of setUI can be
either a string or an object.

libs/js/banglejs/Bangle_setUI_Q3.js

@@ -100,8 +100,13 @@
   }
   if (options.btn) {
     if (Bangle.btnWatches) Bangle.btnWatches.forEach(clearWatch);
+    var e = "falling";
+    if ("object"==typeof options.btn) {
+      e = options.btn.edge;
+      options.btn = options.btn.fn;
+    }
     Bangle.btnWatches = [
-      setWatch(function() { options.btn(1); }, BTN1, {repeat:1,edge:"falling"})
+      setWatch(function() { options.btn(1); }, BTN1, {repeat:1,edge:e})
     ];
   } else if (options.clock) {
     Bangle.btnWatches = [