Show more information in Vala symbol tooltip

[jeremypw]

Fixes #1410

• Extract the source definition from the document and use as tooltip
• Improve the tooltip for the root item
• Fix some consequential timing issues on restoring documents with the outline open
• Fix thread safety and yield to main thread while constructing symbol tree to avoid blocking main thread.
• Update some code style of ValaSymbolItem and move some code from ValaSymbolItem into the construct clause

It proved simpler and better to copy the definition from the document rather than trying to reconstruct from the parsed symbols. The tooltip includes text to the end of the first line pointed to by the symbol.source_reference as this may contain extra useful information such as parameters and/or comments.

[jeremypw]

Getting crash when starting up with certain documents/conditions although opening same documents from sidebar does not cause problems - diagnosing ...

[jeremypw]

Looks like the crashing was due to accessing the source view from a separate parsing thread without locking it. This is now fixed. The parsing thread is now made to yield to the main thread after each child construction and this allows other documents to load and the UI to be used even though there is a document (e.g. a vapi) that takes a long time to finish parsing. Because of this the time allowed to construct the symbol pane has been increased.