server: don't wait anymore for libervia app to be fully started: following change in backend, libervia app are started and the loading workflow continue immediately, the proxy is created only when the app is known to be actually started (through the `application_started` signal or a flag received when starting the application). This avoid stopping the loading of website for a long time, or breaking when a timeout is reached.

import os.path

version_file = os.path.join(os.path.dirname(__file__), "VERSION")
with open(version_file) as f:
    __version__ = f.read().strip()