Allow different DevEnv versions in single Visual Studio project

[doanamo]

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- Add parameter to specify Visual Studio version for generated solution file separately
- If no override is specified, solution file will target the highest Visual Studio version from its configurations

Depends on pull request #203.

Change basically removes/tweaks few checks that prevented having multiple generators output to same project file for Visual Studio, where having projects targeting different Visual Studio versions under single solution is supported. This appears to work quite well without need for any additional changes to sln/vcxproj generators.

This also works under FastBuild with a single difference - for MSBuild you may have to add older build tools components via Visual Studio Installer for solution DevEnv that will be used, where FastBuild will find and directly use build tools from existing installations of older Visual Studio editions.