Suggest to rebuild project in case of code changes

[ForNeVeR]

Sometimes, users change the code outside their XAML, and expect the previewer to take these changes into account.

In those cases, DLL rebuild is required, and the plugin doesn't help people to determine that.

I think we could do better.

1. We can just watch over the .cs file changes and suggest project rebuild if needed.
2. We can also instrument the run using dotCover to better understand what exact lines of code are used in the project and what aren't, and only suggest rebuild if the changes actually affect the currently shown preview.