Fix for #14922

This adds support for F# projects upgraded from VS2012 to VS2013:
* Having properties defined in Choose elements used in Import elements
It also adds partial support for C# and F# projects create in VS2013 (not upgraded from previous version)
* Support for ToolsVersion="12.0"
* Support for comparing conditions comparing numbers between quotes (eg: '$(VisualStudioVersion)' >= '12.0')
* Support for the ImportGroup tag