server: gather themes data: - the new LiberviaRootResource's `site_themes` attribute is a set of available themes for a site, it is set a template variable for pages. - if `browser_meta` are available for a theme, the metadata are merged with pages root `_browser` data - if python files are found in `browser_path`, they are added to be handled by brython task.

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"