Show 'update' instead of uninstall or install for addons which can be updated (maybe we need a third button)
Add a special icon in the addons lists if the addon can be updated
git-svn-id: svn+ssh://svn.code.sf.net/p/supertuxkart/code/main/trunk@5926 178a84e3-b1eb-0310-8ba1-8eac791a3b58