Update .csproj projects to use .shproj

[intvsteve]

Visual Studio 2013 introduced support for Shared Code Projects. These may prove to be extremely helpful in the management of the project sources. Consider the workflow for most of the components in VINTage. Adding a source file typically consists of:

• Add new source file to Foo.flavor.csproj
• Subsequently, this file must be added to all the other flavor projects; the flavors are:
  • xp: Windows xp target (.NET 4.0)
  • desktop: Windows Vista and later target (.NET 4.5)
  • Mac: MonoMac 32-bit macOS target using old version of Xamarin Studio (.NET 4.5)
  • XamMac: Xamarin.Mac 64-bit macOS target using Xamarin Studio (.NET 4.5)
  • VSMac: Xamarin.Mac 64-bit macOS target using Visual Studio for Mac (.NET 4.5)
  • Gtk: Linux Desktop targets using MonoDevelop (.NET 4.5)
• Although one Portable Class Library exists (INTV.Core), it is only shared between the Windows and Linux targets (sorry, Mac)

By adopting the use of Shard Code Projects, much of the project source file management wrangling is eliminated. The need for separate projects is not eliminated, but the process of adding a source file to a build is greatly simplified. That is, rather than adding the same file to up to six projects (though xp, 32-bit macOS, and Xamarin Studio may be dropped), only one project needs modification. Policy regarding where partial class implementation files should reside would of course need to be established.