vtl: prepare simple/advanced usage modes

"simple": localhost only system; hide much of the setup complexity
"advanced":' allow for remote-server, paths mapping, server index etc.
vtl: rc config dialog for simple/advanced config
vtl: simple/localhost mode.

Hide most of the setup complexity that is irrelevant when
running the video-server on the same host as Ardour.

Add an abstraction layer to the config which will become
important later when /building in/ the video-server.