Make data version part of etag

We might add fields to things like file entries (e.g. "display") and in
such cases want to refetch the list even if the files itself didn't
change, to make sure those new fields are available to the clients.

See https://github.com/foosel/OctoPrint/issues/2111#issuecomment-337164940

src/octoprint/server/api/files.py

@@ -29,6 +29,8 @@ import threading
 _file_cache = dict()
 _file_cache_mutex = threading.RLock()
 
+_FILES_DATA_VERSION = 1
+
 def _clear_file_cache():
 	with _file_cache_mutex:
 		_file_cache.clear()
@@ -71,6 +73,7 @@ def _create_etag(path, filter, recursive, lm=None):
 		return None
 
 	hash = hashlib.sha1()
+	hash.update(str(_FILES_DATA_VERSION))
 	hash.update(str(lm))
 	hash.update(str(filter))
 	hash.update(str(recursive))