Move neovimEvent signal to Neovim class
Fix msgpack conversion bug for QPoint type
Fix serialization for QVariantMap

- We can now pass QVariants with QStrings in them in practice this
  is a requirement for QVariantMap, because Qt uses QStrings as keys
- There was a bug with QVariantMap encoding, being serialized
  improperly
- Added new unit test for QVariantMap
- Updated old unit test to reflect new behaviour for strings
Be more strict about QVariant usage

- Implement checkVariant() to verify if a QVariant can be passed onto
  the API
- NeovimConnector::send() will replace an invalid QVariant with Nil
- Encode Nil as an invalid QVariant
- Added unit test for checkVariant()