server: fixed watching a directory which is already watched: Twisted file watched is not adding a callback to a watched path when there is already one, but this can happens often in Libervia, as a whole site is recusively watched in dev mode, and tasks may want to add other watchers. To work around this, the new `_checkCallback` will recursively check that callback has been indeed added to the requested path.

syntax: glob
*.pyc
*.pyv
*.swp
*.swo
*.js
tags
twistd.log
twistd.pid
sat.egg-info
*.un~
dist
MANIFEST
*.sh
build/
Session.vim
ctags_links/
html/