remote-desktop: Accept ConnectToEIS only with started sessions

The ConnectToEIS() method allows the client to communicate directly
with EIS via a socket.

But we need to allow for this only with a fully started session,
otherwise ConnecToEIS() would allow to bypass the permission and
the user dialog entirely.

data/org.freedesktop.portal.RemoteDesktop.xml

@@ -400,7 +400,7 @@
         This method may only be called once per session, where the EIS
         implementation disconnects the session should be closed.
 
-        This method must be called before #org.freedesktop.portal.RemoteDesktop.Start()
+        This method must be called after #org.freedesktop.portal.RemoteDesktop.Start()
 
         Once an EIS connection is established, input events must be sent exclusively via
         the EIS connection. Any events submitted via NotifyPointerMotion,

src/remote-desktop.c

@@ -1407,13 +1407,18 @@ handle_connect_to_eis (XdpDbusRemoteDesktop *object,
 
   switch (remote_desktop_session->state)
     {
+    case REMOTE_DESKTOP_SESSION_STATE_STARTED:
+      break;
     case REMOTE_DESKTOP_SESSION_STATE_INIT:
     case REMOTE_DESKTOP_SESSION_STATE_DEVICES_SELECTED:
     case REMOTE_DESKTOP_SESSION_STATE_SELECTING_DEVICES:
     case REMOTE_DESKTOP_SESSION_STATE_SOURCES_SELECTED:
     case REMOTE_DESKTOP_SESSION_STATE_SELECTING_SOURCES:
-      break;
-    case REMOTE_DESKTOP_SESSION_STATE_STARTED:
+      g_dbus_method_invocation_return_error (invocation,
+                                             G_DBUS_ERROR,
+                                             G_DBUS_ERROR_FAILED,
+                                             "Session is not ready");
+      return G_DBUS_METHOD_INVOCATION_HANDLED;
     case REMOTE_DESKTOP_SESSION_STATE_STARTING:
       g_dbus_method_invocation_return_error (invocation,
                                              G_DBUS_ERROR,