Dev/build tools: Use grunt-autoprefixer for CSS vendor prefixes.

We'll be using it for two distinct tasks:
 * Core CSS files will keep prefixes. `grunt autoprefixer:core` will update files directly in src/ as a pre-commit step, rather than doing it on build.
 * Color CSS files will receive prefixes when they are built.

This commit:
 * Adds prefixes we were missing to core CSS.
 * Removes prefixes that we no longer need from core CSS.
 * Removes all prefixes from colors CSS.

props ocean90.
fixes #27078.

Built from https://develop.svn.wordpress.org/trunk@27174


git-svn-id: http://core.svn.wordpress.org/trunk@27038 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/css/wp-pointer-rtl.css

@@ -5,7 +5,7 @@
 	background: #fff;
 	border: 1px solid #dfdfdf;
 	-webkit-box-shadow: 0 3px 6px rgba(0,0,0,0.075);
-	box-shadow:         0 3px 6px rgba(0,0,0,0.075);
+	box-shadow: 0 3px 6px rgba(0,0,0,0.075);
 }
 
 .wp-pointer-content h3 {
@@ -22,6 +22,7 @@
 
 .wp-pointer-content h3:before {
 	background: #fff;
+	-webkit-border-radius: 50%;
 	border-radius: 50%;
 	color: #2ea2cc;
 	content: '\f227';