Make GS 3.10 compatible

`Main.panel._appMenu` is now `Main.panel.statusArea["appMenu"]`. This API change removes support for GS < 3.10. This can be circumvented by wrapping the calls in conditionals like this:

    // the API changed between 3.8 and 3.10
    const ShellVersion = imports.misc.config.PACKAGE_VERSION;

    if (ShellVersion.search("3.8") == 0) {
        ...
    } else if (ShellVersion.search("3.10") == 0) {
        ...
    }