Add XamlCSS classes to global WPF xml namespace for cleaner syntax

[ThomasPrioul]

Hi, I recently discovered your project and I really like it! I wondered if it wouldn't be cleaner to include the namespaces in the global one, this allows to use (amongst others) the Css class and its attached properties, without using the xmlns prefix all the time (which can be too clumsy IMO).

I added the System.Windows.Markup.ContentProperty attribute which allows not to explicitely put the StyleSheet.Content tag. Also there was a typo with the "Initialize" method missing the last "i", I fixed this. Also VS2019 complained about obsolete tags in csproj files (license and icon url) so I fixed it too. Feel free to just take these little bits and ignore the global tag thing. I thought it could be nice, maybe make an issue about it? Cheers and nice project!