Make it error to use DotvvmView instead of DotvvmMarkupControl

[exyi]

When the user writes a dotvvm markup control with .dothtml extension and doesn't use @baseType directive, DotVVM will use DotvvmView as a base type. When the control is then initialized, there are multiple DotvvmViews in the page.

Easier solution is to make it an error to declare a markup control in such way. It's obviously breaking, so milestone 5.0

Alternative would be to use DotvvmMarkupControl as default base type, whenever the page is registered in DotvvmMarkupConfiguration. This is breaking too, if someone is using the page as view and also declared it as a control.