dropdown: Make the popup as wide as the button

Having narrow popups on a wide button looks awkward.
Do the same as the combo box, and ensure the popup
is at least as wide as the button.

gtk/gtkdropdown.c

@@ -396,6 +396,8 @@ gtk_drop_down_size_allocate (GtkWidget *widget,
 
   gtk_widget_size_allocate (self->button, &(GtkAllocation) { 0, 0, width, height }, baseline);
 
+  gtk_widget_set_size_request (self->popup, width, -1);
+
   gtk_native_check_resize (GTK_NATIVE (self->popup));
 }