Introduce a .dashicons-before CSS class.

If you want to use a Dashicon before an element you can use the class because it will not change the elements content styling. With that you don't need to copy the entire `.dashicons` styling to your :before styling anymore.
To demonstrate this change, Admin Menu now uses Dashicons classes directly.

And it fixes a glitch in IE.

fixes #26630.
Built from https://develop.svn.wordpress.org/trunk@27418


git-svn-id: http://core.svn.wordpress.org/trunk@27265 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/css/dashicons.css

@@ -12,11 +12,11 @@
 	font-style: normal;
 }
 
-.dashicons {
+.dashicons,
+.dashicons-before:before {
 	display: inline-block;
 	width: 20px;
 	height: 20px;
-	-webkit-font-smoothing: antialiased;
 	font-size: 20px;
 	line-height: 1;
 	font-family: "dashicons";
@@ -24,12 +24,13 @@
 	font-weight: normal;
 	font-style: normal;
 	vertical-align: top;
-	-moz-transition: color .1s ease-in 0;
-	-webkit-transition: color .1s ease-in 0;
 	text-align: center;
+	-webkit-transition: color .1s ease-in 0;
+	transition: color .1s ease-in 0;
+	-webkit-font-smoothing: antialiased;
+	-moz-osx-font-smoothing: grayscale;
 }
 
-
 /* Admin Menu Icons */
 
 .dashicons-menu:before {