Add StatusArea widget

A palceholder, for now.

src/status_area.rs

@@ -0,0 +1,56 @@
+use cascade::cascade;
+use gtk4::{
+    glib,
+    prelude::*,
+    subclass::prelude::*,
+};
+
+use crate::deref_cell::DerefCell;
+
+#[derive(Default)]
+pub struct StatusAreaInner {
+    box_: DerefCell<gtk4::Box>,
+}
+
+#[glib::object_subclass]
+impl ObjectSubclass for StatusAreaInner {
+    const NAME: &'static str = "S76StatusArea";
+    type ParentType = gtk4::Widget;
+    type Type = StatusArea;
+
+    fn class_init(klass: &mut Self::Class) {
+        klass.set_layout_manager_type::<gtk4::BinLayout>();
+    }
+}
+
+impl ObjectImpl for StatusAreaInner {
+    fn constructed(&self, obj: &StatusArea) {
+        let box_ = cascade! {
+            gtk4::Box::new(gtk4::Orientation::Horizontal, 0);
+            ..set_parent(obj);
+        };
+
+        self.box_.set(box_);
+    }
+
+    fn dispose(&self, _obj: &StatusArea) {
+        self.box_.unparent();
+    }
+}
+
+impl WidgetImpl for StatusAreaInner {}
+
+glib::wrapper! {
+    pub struct StatusArea(ObjectSubclass<StatusAreaInner>)
+        @extends gtk4::Widget;
+}
+
+impl StatusArea {
+    pub fn new() -> Self {
+        glib::Object::new(&[]).unwrap()
+    }
+
+    fn inner(&self) -> &StatusAreaInner {
+        StatusAreaInner::from_instance(self)
+    }
+}