A prototype of inline completion

[jlahoda]

This attempts to add a support for inline completion to NetBeans. It adds a support to add "shadow" text (including multiple lines), and enhances the LSP client to use it (if the server supports it, of course).

Can be tested with this server:
https://github.com/jlahoda/inline-completion-test-server

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[mbien]

if this can't be javac.release, please add a comment why not, otherwise someone might break this in future.