Add Attr.DynamicClassPred

[granicz]

We currently have Attr.DynamicClass to set a CSS class when a view satisfies a given predicate. This could benefit from an Attr.DynamicClassPred variant that takes a View<bool> instead of the View<'T> and the 'T -> bool predicate.

[Tarmil]

Here's the full list of changes I intend to do:

• Add the following:

  val DynamicClassPred : name: string -> isSet: View<bool> -> Attr
  
  // V-enabled on `isSet`, which makes it compile into DynamicClassPred
  val ClassPred : name: string -> isSet: bool -> Attr

• Mark the current DynamicClass as obsolete, pointing to ClassPred and DynamicClassPred as replacements.