Improving .select2-hidden-accessible

To match last standards, see [CSS hide and seek](http://hugogiraudel.com/2016/10/13/css-hide-and-seek/) for more explanations :)

Also I removed the negative margin because of unwanted effects in some case, as seen on Bootstrap.

src/scss/core.scss

@@ -36,12 +36,14 @@
 .select2-hidden-accessible {
   border: 0 !important;
   clip: rect(0 0 0 0) !important;
+  -webkit-clip-path: inset(50%) !important;
+	clip-path: inset(50%) !important;
   height: 1px !important;
-  margin: -1px !important;
   overflow: hidden !important;
   padding: 0 !important;
   position: absolute !important;
   width: 1px !important;
+  white-space: nowrap !important;
 }
 
 @import "theme/default/layout";