Changing a weblink's title is not reflected where the weblink filter is used

After changing the title of a weblink node, the old title remains in all instances where the [weblink] tag has been used. This is because filter results are cached.

This patch solves the problem by clearing the cache entries for all input formats, potentially clearing many more entries than needed. Would it be worth finding which input format has the weblink filter enabled and then clearing only those?