NewCallBox+CallDisplay: UI facelift

This adds a style.css file to allow the use of custom css

Fixes https://source.puri.sm/Librem5/calls/issues/115:
- Making the phone number larger
- Moving the delete button to the bottom
- Rounding all buttons, and making the grid buttons circular
- Making the call button a bit less wide

src/calls-application.c

@@ -158,6 +158,28 @@ static const GActionEntry actions[] =
 };
 
 
+static void
+css_setup ()
+{
+  GtkCssProvider *provider;
+  GFile *file;
+  GError *error = NULL;
+
+  provider = gtk_css_provider_new ();
+  file = g_file_new_for_uri ("resource:///sm/puri/calls/style.css");
+
+  if (!gtk_css_provider_load_from_file (provider, file, &error)) {
+    g_warning ("Failed to load CSS file: %s", error->message);
+    g_clear_error (&error);
+    g_object_unref (file);
+    return;
+  }
+  gtk_style_context_add_provider_for_screen (gdk_screen_get_default (),
+                                             GTK_STYLE_PROVIDER (provider), 600);
+  g_object_unref (file);
+}
+
+
 static void
 startup (GApplication *application)
 {
@@ -175,6 +197,8 @@ startup (GApplication *application)
                                    actions,
                                    G_N_ELEMENTS (actions),
                                    application);
+
+  css_setup ();
 }