Enable sending configuration changes to the language server

[winstliu]

The Eclipse language server contains many preferences that the client can configure to control its behavior. Up until now, there was no way to change any of those preferences. With this PR, I've:

1. Restructured the package settings to match the structure that the language server expects
2. Hooked into the relevant atom-languageclient functions so that
   • settings are passed to the language server on load, and
   • the language server receives updated configurations whenever ide-java settings change
3. Removed the special-casing for the incomplete classpath notification, now that we can properly configure the language server to not send a notification
4. Exposed the enabling of signature help as an option (and fixed it so that it actually works in the first place)

[winstliu]

This paves the way for many future enhancements, for example properly handling build configuration notifications (which I plan to send a PR for next).

[winstliu]

/cc @Aerijo just wondering if you have a better idea for the overarching setting group than "server".

[Aerijo]

wondering if you have a better idea for the overarching setting group than "server"

It's in the settings page, and it's for the server. That heading seems perfectly adequate.