Migrating to the slnx format

[lipchev]

• UnitsNet.sln -> UnitsNet.slnx
• Samples/Samples.sln -> Samples/Samples.slnx
• Samples/MvvmSample.Wpf/MvvmSample.Wpf.sln -> Samples/MvvmSample.Wpf/MvvmSample.Wpf.slnx
• Samples/UnitConverter.Wpf/UnitConverter.Wpf.sln -> Samples/UnitConverter.Wpf/UnitConverter.Wpf.slnx

and updating the build functions that reference UnitsNet.sln

[lipchev]

@angularsen I couldn't find any information about what to do with the sln.DotSettings. I tried renaming it but I couldn't figure out if that changed anything: I tried creating a new class in the UnitsNet project but neither had the copyright-header-tag. I then tried to do it in the old version and again- no file header.. I don't know what's up with that, please check on your end..

By the way, In the Manage Settings this is still referred to as a "sln.DotSettings file..".