Request: Placeholder support for the textbox control

This is a future feature request for the textbox control (and maybe others as well where this fits into) to have easy property support for a watermark / placeholder text. This is one of the most things you see in custom control WPF libraries. It should be easy to add to the source.

Please note that watermark also has been added to the winforms repo already https://github.com/dotnet/winforms/pull/238 in case you may ask about the why. I could submit a PR but I'm not sure if that's too early as the WPF team may be too busy to work on the .NET Core 3 release for September?