[xbuild] Build & install MSBuild 12.0 assemblies
[xbuild] Install data files for xbuild 12
[xbuild] Make xbuild script point to xbuild 12
[xbuild] Update printed version/copyright
[xbuild] MSBuild 12 toolset should be available for XBUILD_12
[xbuild] Fix 12.0 tasks
[xbuild] Fix 12.0 toolset location and public API
[xbuild] Set correct DefaultToolsVersion for xbuild 12
[xbuild] Remove Silverlight targets, they don't work w/o framework
[xbuild] Add binding redirects so old tasks work w/ new xbuild
[xbuild] Fix the xbuild 12 tests
[xbuild] Add pkgconfig file for xbuild 12
[xbuild] Suppress new window for child processes

BXC14584 - Console windows pop up during build with mono runtime