Improve signals and rendering

[wartman]

Right now it's possible to create signals inside the render method of components which requires the use of an "Isolate" to clean things up. This seems bad.

Instead of this, we should introduce a dev-mode check that throws at runtime if a signal is created inside a render method. We can then remove the Isolate class both from the Component implementation and from the entire codebase. The setup method and signal metadata (@:computed etc) can handle the rest.