…ui-error package

This introduces a very basic implementation of the observable pattern
for the Neos UI.

This will allow us to replace redux opportunistically in various places.

This new package contains react bindings for the other recently
introduced package `@neos-project/framework-observable`.

...by using the newly introduced observable primitives. The
neos-ui-error package now exposes a method to show flash messages.

A saga has been created to redirect redux actions (that may still be
used in plugins) to that method.

…owFlashMessage` API