Only append data on completion item selected

[jdneo]

• Instead of adding data to the completion items directly, now we only append those data when 'onDidSelect' happens. Because those data are only used by the registered ranking providers. Both server and client do not care about it actually.

[jdneo]

Make sure the IntelliCode is installed and then triggering completion at | in the following snippet:

public static void main(String[] args) {
    args.|
}

The response size reduced from 22.2KB to 15.8KB.

If completion after a string: args[0].|

Then response size reduced from 146KB to 83.7KB

[rgrunber]

Ok, so based on https://github.com/eclipse/eclipse.jdt.ls/pull/2639/files#diff-335831335331faa697d0e108634d599cf7cddd65328987dd8047572fd233b55dR96-R100 the data field was never meant for use by the client. If that's the case, then this seems fine to me.