Update version to 0.8.0.

fix release branch detection.

Merge pull request AvaloniaUI#1379 from jkoritzinsky/refcount-scene

Make Scene own VisualNodes

Removed NUGET_API_KEY from appveyor.yml

Merge pull request AvaloniaUI#1001 from kekekeks/release-branches

Allow publishing to nuget from non-release branches

Updated version to 0.5.0.

0.4.0 Alpha 4

Updated nuget version

Merge branch 'alpha1'