Gvls plugin

[jeremypw]

Simplified version based on #599 and using GVLs v 20

Required building and installation from source https://gitlab.gnome.org/esodan/gvls/-/tree/gvls-20 libgvls libgvlsp libgvlsui

Currently builds and runs and connects to server but no completions provided - terminal shows error message

gvlsui-project-manager.vala:93: Error while pushing changes to the server: No stream available to deliver invocation

[jeremypw]

@esodan Anything obvious why this does not work? I haven't had time yet to dig through the library code. I was wondering whether it would be better just to get a list of candidates from the server and populate the completion ourselves in the same way the word completion and snippet plugins work. I do not really like passing the sourceview to an external library.