Correctly v-center the widgets

src/window.rs

@@ -124,6 +124,7 @@ impl Window {
     fn setup_widgets(&self) {
         let container = gtk::Box::new(gtk::Orientation::Vertical, 0);
         container.get_style_context().add_class("main-container");
+        container.set_valign(gtk::Align::Center);
         container.show();
 
         self.fg_entry.entry.get_style_context().add_class("fg-entry");
@@ -143,8 +144,8 @@ impl Window {
         entries_container.show();
         self.headerbar.set_custom_title(Some(&entries_container));
 
-        container.pack_start(&self.preview.borrow().widget, true, true, 0);
-        container.pack_start(&self.levelbar.borrow().widget, false, false, 0);
+        container.pack_start(&self.preview.borrow().widget, true, true, 18);
+        container.pack_start(&self.levelbar.borrow().widget, false, false, 18);
 
         self.widget.add(&container);
         // Restore state