Added fix to migrate old F# project guid to the correct one.

When the project is saved the old guid is is updated to the new one.
Existing project with the old guid continue to work as the project
loader recognises the F# file extension.
Added CorrectGuidMSBuildExtension type to deal with the upgrade
Merge pull request #580 from 7sharp9/guidfixes

Added fix to migrate old F# project guid to the correct one.