PlaybackManager: create actions internally

[danirabbit]

Setup playbackmanager actions inside the playbackmanager. Makes sure playback manager functions are private so they're only used through actions. Manage state internally. Simplify setup

[danirabbit]

@ryonakano Should I move the action constants to PlaybackManager as well? I wasn't sure if those should stay all centralized or be with the action creation

[ryonakano]

@danirabbit Hmm it's difficult to judge, but maybe they would be better to be kept in Application?

These constants themselves are no longer used in Application but the actions invoked by using these constants are still registered to it. So thinking about where we use them, I feel the former (current) code makes sense and hide internal implementation that the handler of the actions are actually no longer in Application but in PlaybackManager:

        // example 1
        var play_button = new Gtk.Button () {
            action_name = Application.ACTION_PREFIX + Application.ACTION_PLAY_PAUSE,
            child = play_pause_image,
            halign = Gtk.Align.CENTER,
        };

        // example 2
        var play_pause_action = GLib.Application.get_default ().lookup_action (Application.ACTION_PLAY_PAUSE);
        play_pause_action.bind_property ("enabled", seekbar, "sensitive", BindingFlags.SYNC_CREATE);

rather than:

        // example 1
        var play_button = new Gtk.Button () {
            // I think this implicitly make the users of this action, us programmers, to be aware of the internal implementation
            action_name = Application.ACTION_PREFIX + PlaybackManager.ACTION_PLAY_PAUSE,
            child = play_pause_image,
            halign = Gtk.Align.CENTER,
        };

        // example 2
        // Why do we use the constants of PlaybackManager to lookup actions in Application……?
        var play_pause_action = GLib.Application.get_default ().lookup_action (PlaybackManager.ACTION_PLAY_PAUSE);
        play_pause_action.bind_property ("enabled", seekbar, "sensitive", BindingFlags.SYNC_CREATE);

[ryonakano]

Confirmed these actions still works. One small nitpicking otherwise LGTM.

[ryonakano]

Thanks for the cleanup! 🧹 ✨