Add tooltips to members from XML documentation

[manio143]

Reading the XML doc format is explained here: https://docs.microsoft.com/en-us/archive/msdn-magazine/2019/october/csharp-accessing-xml-documentation-via-reflection

We can make a similar class to this: https://github.com/stride3d/stride/blob/master/sources/editor/Stride.Core.Assets.Editor/Services/UserDocumentationService.cs

Then add public string Tooltip { get; } to the MemberViewModel, which in its ctor would get and call DocumentationService.
Note: this will impose a dependency on IServiceRegistry across all view models that create members.