Remove Blockly.removeAllRanges and replace it with calls to add/remove

css to mark text as unselectable instead.  I can't prove this is
significantly faster with numbers since the call to setup the time out
costs ~.1ms and the call when the timer fires is ~.05.  This happens on almost
every mouse event though.  Plus, it seems to remove a flicker of highlighting when
frames are skipped and keeps scrolling via the scrollbar from highlighting.

core/css.js

@@ -143,6 +143,13 @@ Blockly.Css.CONTENT = [
     'z-index: 999;',
   '}',
 
+  '.blocklyNonSelectable {',
+    'user-select: none;',
+    '-moz-user-select: none;',
+    '-webkit-user-select: none;',
+    '-ms-user-select: none;',
+  '}',
+
   '.blocklyTooltipDiv {',
     'background-color: #ffffc7;',
     'border: 1px solid #ddc;',