Expose opened file in the startup of the language server

[jonas-jonas]

Currently there is no way of "attaching" a language server to more than a content type. For Flutter this needs to be done as Flutter ships its own SDK and we need to decide which language server to start up upon opening of a file, which is not possible right now.

Work needs to be done in LSP4E.
I opened https://bugs.eclipse.org/bugs/show_bug.cgi?id=552987 for that.