Add comment property to ContentPlaceHolder for design-time use

[thatdbme]

This is copied from https://forum.dotvvm.com/t/feature-request-suggestion/119

It would be helpful if the dot:ContentPlaceHolder had some kind of comment property that displayed the value on the dot:Content control on the page at design time. I would envision this having no effect on the page content sent to browsers. I’m not sure the best way to accomplish that. Maybe a VS tooltip that appeared when the dot:Content control is hovered over after selecting the dot:ContentPlaceholderID.

This would allow page developers to see the purpose/description of any given placeholder without having to follow it back to the master page or look at other examples.