Disable selection of the icons

When double clicking or trying to select text manually you might
accidentally select icons which might have a non desired character.

This commit disables such selections by using the non standard CSS
propery user-select:
https://developer.mozilla.org/en-US/docs/CSS/user-select
as well as -webkit-touch-callout:
http://css-infos.net/property/-webkit-touch-callout