templatefilters: use templatefilter to mark a function as template filter Using decorator can localize changes for adding (or removing) a template filter function in source code. This patch also removes leading ":FILTER:" part in help document of each filters, because using templatefilter makes it useless. This patch uses not 'filter' but 'templatefilter' as a decorator name, because the former name hides Python built-in one, even though the latter is a little redundant in 'templatefilters.py'.