Get rid of `identifier` requirement of `App` protocol

[stackotter]

The identifier property is only used by Gtk so I reckon GtkBackend should just use a default value and provide some way to configure it. Alternatively, we could just provide a default implementation of the identifier property which has some default identifier such as com.example.Example (not sure what a good default would be).

[warmachinesocial]

Did you mean this under GtkBackend.swift

    public typealias Window = Gtk.Window
    public typealias Widget = Gtk.Widget

    var gtkApp: Application

    /// A window to be returned on the next call to ``GtkBackend/createWindow``.
    /// This is necessary because Gtk creates a root window no matter what, and
    /// this needs to be returned on the first call to `createWindow`.
    var precreatedWindow: Window?

    public init(appIdentifier: String) {
        gtkApp = Application(applicationId: appIdentifier)
    }

[stackotter]

Yeah, that initialiser is currently the only piece of code that actually uses the appIdentifier for anything, the other backends just ignore it, so it should be a gtk specific setting instead of a setting provided to all backends.

[stackotter]

Completed this on the layout_system branch a little while ago.