Allow users to set startup project

[prashantvc]

User msbuild target to generate previewer parameters. Add previewer target to the project only in debug mode

[lextm]

The solution model (cache) is solution-specific, while the previewer requires a project-specific setting. So, when we work on the fork, we add two new settings and allow users to select both of them. We are considering a third for build configuration (Debug/Release/etc.) and more (primary target framework if the project has many).