Performance: do not draw header bar bottom border

It is not even noticable and reduces performance by having more pixels to redraw every frame.

clapper_src/headerbar.js

@@ -8,6 +8,7 @@ class ClapperHeaderBar extends Gtk.HeaderBar
         super._init({
             can_focus: false,
         });
+        this.add_css_class('noborder');
 
         this.set_title_widget(this._createWidgetForWindow(window));
         let clapperWidget = window.get_child();

css/styles.css

@@ -152,6 +152,9 @@ scale marks {
 .nobackground {
   background: none;
 }
+.noborder {
+  border: none;
+}
 .controlsbox {
   background: @theme_bg_color;
 }