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.

import os.path

version_file = os.path.join(os.path.dirname(__file__), "VERSION")
try:
    with open(version_file) as f:
        __version__ = f.read().strip()
except NotImplementedError:
    # pyjamas workaround
    __version__ = "0.7.0D"