Fix creating QWebChannel on first QWebEnginePage load

The "qt" object may be undefined at the time QWebEngineScript
that creates web channel is being run. Try to create the web channel
until it finally succeeds in 100ms intervals.

Fixes the "Uncaught ReferenceError: qt is not defined" error message.

src/lib/tools/scripts.cpp

@@ -45,9 +45,16 @@ QString Scripts::setupWebChannel()
                            "    return;"
                            "}"
                            ""
+                           "function registerWebChannel() {"
+                           "    try {"
                            "        new QWebChannel(qt.webChannelTransport, function(channel) {"
                            "            registerExternal(channel.objects.qz_object);"
                            "        });"
+                           "    } catch (e) {"
+                           "        setTimeout(registerWebChannel, 100);"
+                           "    }"
+                           "}"
+                           "registerWebChannel();"
                            ""
                            "})()");