Make joystick window always-on-top to avoid Chrome scheduling penalty

Chrome severely reduces the interval of setInterval() calls when a
window is completely hidden (e.g. behind the main Configurator window).
This causes rx loss / failsafe detection to trigger when using the
RX_MSP joysticks

manifest.json

@@ -31,7 +31,8 @@
         "fileSystem",
         "fileSystem.write",
         "fileSystem.retainEntries",
-        "notifications"
+        "notifications",
+        "alwaysOnTopWindows"
     ],
 
     "optional_permissions": [

tabs/receiver.js

@@ -324,7 +324,8 @@ TABS.receiver.initialize = function (callback) {
                     minWidth: windowWidth, minHeight: windowHeight,
                     width: windowWidth, height: windowHeight, 
                     maxWidth: windowWidth, maxHeight: windowHeight
-                }
+                },
+                alwaysOnTop: true
             }, function(createdWindow) {
                 // Give the window a callback it can use to send the channels (otherwise it can't see those objects)
                 createdWindow.contentWindow.setRawRx = function(channels) {