Fixed positioning for the text selection button in Firefox when the page is scrolled

git-svn-id: https://plugins.svn.wordpress.org/quotable/trunk@899385 b8457f37-d9ea-0310-8a92-e5e31aec5664
update changelog

git-svn-id: https://plugins.svn.wordpress.org/quotable/trunk@899387 b8457f37-d9ea-0310-8a92-e5e31aec5664