Added filesApp flag as input field for files app detection

Since the files app can be reached under multiple URLs, either root,
files.php or public.php, a flag has been added to the DOM to help apps
(like file viewers) to detect whether they are currently in the files
app.

@karlitschek @DeepDiver1975 @schiesbn please review.

This is needed to fix the PDF viewer app (#1409)