Initial support for API events

- We now parse notification event messages from Neovim
- Added an example on how to trigger an event
- We are still missing an API on register an event handler
Add API for event handling

- This commit defines the API for handling Neovim events,
  providing two ways to receive events
- New method setNeovimEventHandler to define QObject whose slots
  are called on events
- New signal notification() catches ALL notifications however the
  arguments are passed as a QVariant - this is more usefull for
  debugging or internal use since you still have to decapsulate
  the varuant.
- Updated example