cargo: Remove gtk-macros dependency

Using the crate is depreacted. Replaced spawn! macro
with our own spawn function

src/util.rs

@@ -198,3 +198,10 @@ pub fn contrast_ratio(color1: &gdk::RGBA, color2: &gdk::RGBA) -> f32 {
 
     (l1 + 0.05) / (l2 + 0.05)
 }
+
+pub fn spawn<R: 'static, F: std::future::Future<Output = R> + 'static>(
+    f: F,
+) -> glib::JoinHandle<R> {
+    let ctx = glib::MainContext::default();
+    ctx.spawn_local(f)
+}