An alternate (and less expensive) way to keep stuff outside of the blockly div

from getting selected. Use e.preventDefault instead of adding and removing
classes on mousedown and mouseup.  This keeps the browser from having to
potentially recacluate style on mousedown and mouseup events.

core/css.js

@@ -143,13 +143,6 @@ 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;',