Language servers will now receive an open file instead of just its path. This
means the language servers no longer need to access the filesystem to open the
file themselves.
The C++ language server now has no filesystem access whatsoever (although we
might need to relax this in the future if it learns to complete #include paths),
while the Shell language server can read /etc/passwd (it wants that in order to
get the user's home directory) and browse (but not read!) the whole file system
tree for completing paths.
The language server keeps track of the content of currently edited
files by receiving updates about edit actions.
Also, C++ autocompletion is no longer tied to HackStudio itself and
moved to be part of the language server.
