[Gtk-3.20] Fix Combobox menu padding. Fixes #575

Author: megax

src/gtk-3.20/scss/widgets/_menu.scss

@@ -45,9 +45,9 @@
     menu,
     .menu,
     .context-menu {
-        padding: 0;
-        border-radius: 0;
         border: 0;
+        border-radius: 0;
+        padding: 2px 0;
         background-color: $menu_bg_color;
         color: $menu_fg_color;