Two new convenience functions in lib/Theme.php: WikiLink() and Button().

Two new convenience functions in lib/Theme.php: WikiLink() and Button().
Most invocations of $Theme->link*WikiWord(), and $Theme->make*Button()
can be replaced by calls to one of these new function.
It helps beautify the templates quite a bit.

git-svn-id: svn://svn.code.sf.net/p/phpwiki/code/trunk@1719 96ab9672-09ca-45d6-a79d-3d69d39ca109