added basic support for setting a main file in  a pvsio-web project.
Main file is set in a file in the root of the project called ".pvsioweb". The file is a JSON key value pair containing {"mainPVSFile": "<filename>"}
If there is not .pvsioweb file, then the main pvsfile is assumed to be the last file when all the pvs files are sorted alphabetically.

TODO is support on the client for properly listing and editing all the files.