Index global descriptors and provide completions for unimported symbols

[fwcd]

Fixes #10

This is some experimental work on indexing the project's globally available symbols in the background and offering completions for them, regardless of whether they have been imported or not:

Todo:

• Index globally available symbols on a background thread
  • Store visibility in symbol index
• Offer completions for these symbols (WIP)
  • Filter by visibility (public, package-private, ...)
  • Filter out already imported symbols (this includes implicitly imported ones, e.g. from java.lang, ...)
    • Handle wildcard imports
  • Automatically add the required imports when selected
    • Find a good import position (e.g. by inserting lexicographically)
  • Offer completions for global/non-imported extension methods/properties
• Add a config option for disabling symbol indexing completely (e.g. for saving memory)
• Fix existing code completion unit tests (may require awaiting the initial symbol indexing)
• Index entire project when language server starts up

Additionally, this branch introduces a new Progress abstraction for emitting progress notifications to the client, if supported.

Future ideas, not in the scope of this PR:

• (Store rendered descriptor previews in symbol index #270) Store rendered descriptor previews in symbol index
• (Store location URIs in symbol index #271) Store location URIs in symbol index
• (Store location URIs in symbol index #271) Use symbol index for workspace symbols and go-to-definition (may require URIs in symbol records, see above)
• (Reindex project incrementally as local files change #272) Reindex project incrementally as local files change
• (Reindex entire project when the compiler is reinstantiated #273) Reindex entire project when the compiler is reinstantiated, e.g. due to classpath changes