Allow custom control subclasses to be placed on web pages

A user-defined control subclass such as ChartView Inherits Canvas compiles in a web project but cannot be placed on a web page: the designer rejects it as an "unknown control type", because web page layouts accept only the built-in control types. Today the supported pattern is to place a plain Canvas and forward to the class by hand (call chart.Draw(g, ...) in Paint, forward mouse events), which works but must be repeated on every page and gives up the designer integration.

Requested: allow project-defined control subclasses, starting with Canvas subclasses (custom drawn controls), to be placed on web pages from the designer, so a control like ChartView is designed once and placed anywhere, the same way it already works on desktop.