Fix for #14922

This adds support for F# projects upgraded from VS2012 to VS2013:
* Having properties defined in Choose elements used in Import elements
* Having properties used in Import elements that after expansion use MSBuildExtensionPath

It also adds partial support for F# projects created 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 elements
* Support for * in Import elements
* Add new MSBuild properties