More robust "go to definition"

[cameel]

The feature was implemented in a rush at the hackathon and needs cleanup and improvements:

• We seem to have a duplicate of tower_lsp::lsp_types::TextDocumentItem struct in our code (that also causes warnings due to unused fields). We should switch to using that directly.
• We have no tests covering it.
• We only managed to work with an old version of QtCreator. We need to make it work in all editors.