[FEATURE] Register Custom Widgets

[Byson94]

Description of the requested feature

Register custom widgets using gtk4-rs via the plugin API.

Regarding FFI Concern

Although a plain widget (like gtk4::Box) cannot pass across FFI safely, the underlying GtkWidget can as its #[repr(C)]. The plain widget can be converted into a raw C pointer and passed to ewwii, which it can then reconstruct.

This method preserves the properties and controllers attached to the widget. Which is ideal.

Proposed configuration syntax

A custom widget can be used like this:

Custom {
    name = "widget-name"
    # ... props
}

There is no clean way to register the widgets as MyProgress { .. } because ewwii uses a fixed list of widgets (WidgetNode). This pattern works because Custom can be just another widget in WidgetNode. If this is verbose for the developer writing the widget support plugin, they can use inject_nbcl to make a new component that wraps the custom widget.

Additional context

The custom widgets can also be required a specific structure that is similar to the EwwiiWidget trait system used currently in ewwii as of right now.

[Byson94]

After thinking about this problem for a while, I decided to come up with two API's.

1. register_widget
2. register_static_widget

The first one can be used to register a fully functional widget that can handle rendering its children and handling ewwii properties. This is the more complex one. On the other hand, the second (static) one is simpler. It just tells ewwii to render the widget and ignore everything else including children and properties.

I'll work on the static widget registration API first then move on to the full version as it will abstract some of the complexity away and help be build the foundation first. The only one problem with this way of implementation is that I have to build a unified interface which both static and non-static widgets can use.

[Byson94]

The API of register_static_widget can be fairly simple.

use ewwii_plugin_api::gtk4;

let box = gtk4::Box::default();
let box_general = box.upcast::<gtk4::Widget>();
host.register_static_widget("awesome-widget", box_general);

And this can then be used like so in nbcl:

Custom { name = "awesome-widget" }

Additional Context

The widget registration API's and the reexported gtk4 crate is only accessible if the widgets feature is enabled.

ewwii_plugin_api = { version = "*", features = ["widgets"] }