Ability to override target framework in ElectronDevMode

Electron.NET is targeting multiple frameworks, including deprecated ones.

While using ElectronDevMode we might use it as a submodule of an existing product where only some targets are available.

In order to enhance developer experience, add the ability to overload the target framework from the `Directory.Build.props` barrier.

In addition, this will solve the Visual Studio message '`This project is targeting a version of .NET that is not installed`' for solutions without multiple target frameworks.

First locate ElectronDevMode and the Directory.Build.props barrier, then inspect how target frameworks are currently selected. Done means an existing product can override the target framework when using ElectronDevMode, and Visual Studio no longer reports an unavailable framework for single-framework solutions.

索引モデルが issue の本文から書いたものです。