More in-app documentation

[pckerneis]

I'd like more in-app documentation in various forms.

• The only one implemented today is the tips for Message elements (a popup-up filtered list of methods accessible is shown when the message's editor is used).
• There should be a description of WebAudio elements such as nodes, contexts, buffer... It could be a tip window in the LibraryPanel or maybe a footer bar (but it'd be a small space for potentially long texts).
• There should be a documentation panel. It can be based on JUCE's WebBrowserComponent to show HTML doc. Quick access to topics should be possible (something like "Show in documentation" in graph elements popup menu).
• Auto-completion/validation in the code editors is too ambitious for now. It'd require to actually interpret the JS code.

[pckerneis]

The documentation for WebAudio objects and methods can be adapted from the w3c specs and Mozilla's documentation. The documentation should be clear about what is natively present in WebAudio and the app-specific layer of graph elements (messages, dynamic routes, scripts, DecodableAudio,...).