add support for Structure elements filtering

[dacheng-gao]

It would be great if we can control hide / show inside Structure tool window:

• private and public
• different type of elements: fields, constants, variables, functions

[FalsePattern]

Structure view is implemented inside LSP4IJ, so you'll have to forward this request to them.

[angelozerr]

@dacheng-gao please create an issue at https://github.com/redhat-developer/lsp4ij/issues