Add new assembly Mono.XBuild.Utilities to enable unit testing of internal classes

This change
  * updates csproj files for the MS.Build.* assemblies and xbuild (1st & 3rd commit)
  * adds a new assembly Mono.XBuild.Utilities and moves prev. internal classes from
    Microsoft.Build.Utilities into the new assembly, to make them unit-testable. (2nd commit & 4th commit)

Detailed descriptions can be found in the commit messages.

This patch is MIT/X11 licensed.