Unity version and app path should always be in sync

[van800]

Requirements:

1. path and version used in StartUnity action should be in sync.
2. We have multiple ways of guessing the path and version. Like reading ProjectVersion.txt, EditorInstance.json, inferring from path to Unity*.dll in csproj
3. Inferring from path to Unity*.dll in csproj is good because it helps to handle any custom unpredictable location of Unity installation. But version in ProjectVersion.txt should be preferable.