Use TURF Properties files for site description sidecar files.

There remains the issue of whether to use the form filename.ext._.turf-properties or .filename.ext._.turf-properties for the sidecar file (i.e. whether to make the sidecar file a dotfile). On one hand we want to indicate it's not a normal content file. On the other hand we want to make it easy to move with the original file, and dotfiles aren't easy to copy from the command line in Linux, or even find in the CLI or the GUI at times. So we should err on the side of them not getting lost, which I believe was the reasoning behind not making them dotfiles to begin with.

For now Guise is putting the site description in a separate directory. Perhaps in the future we might consider detecting that the description is being put in the same directory and use dotfiles in that case.

Using _ for the sidecar extension segment could cause confusion with the veiling character (e.g. _hide-from-navigation.html), especially when used for directory metadata: we'd either have to go with _.turf-properties, which looks like a veiled file; or ._.turf-properties, even though may not want dotfiles in a separate site description directory; or something like _._.turf-properties, which starts to get wordy.

Perhaps it's better to go with a - extension segment. This makes filename.ext.-.turf-properties for a normal sidecar, and -.turf-properties for a directory sidecar, which can be made into .-.turf-properties if we need to use dotfiles.

This improvement will be performed as part of GUISE-136.

It looks like the embedded server (specifically SiteDirResourceSet) wasn't updated to recognize the new .-.tupr extension from this ticket. (It has a hard-coded string.) Reopening this ticket to fix that.