[gtkmm/dropdown-demo] DropDown demo: use set_margin

[Andreas Persson <andreasp src gnome org>]

commit 5153aa4d16456f6b0e2e02995fc318756cffaf52
Author: Andreas Persson <andreasp56 outlook com>
Date:   Sun Sep 20 19:59:31 2020 +0200

    DropDown demo: use set_margin
    
    Use set_margin instead of separate calls to set the margins of a box.

 demos/gtk-demo/example_dropdown.cc | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)
---
diff --git a/demos/gtk-demo/example_dropdown.cc b/demos/gtk-demo/example_dropdown.cc
index a96e4c1b..83e2624a 100644
--- a/demos/gtk-demo/example_dropdown.cc
+++ b/demos/gtk-demo/example_dropdown.cc
@@ -239,10 +239,7 @@ Example_DropDown::Example_DropDown()
   set_title("Drop Downs");
   set_resizable(false);
 
-  m_box.set_margin_start(10);
-  m_box.set_margin_end(10);
-  m_box.set_margin_top(10);
-  m_box.set_margin_bottom(10);
+  m_box.set_margin(10);
   set_child(m_box);
 
   auto model = Pango::CairoFontMap::get_default();