StableId generation default fixed seed is problematic, make configurble

[stevegeek]

In the current setup, the sequence of IDs generated for elements is stable, however it always is the same for element types across requests. So for example, if a page generates a page with a form in it, then via Ajax requests another form, which it then attaches to the DOM. They will both have the same ID since it was actually two requests that generated the two forms as the first form elements and they'll both have the same ID. We need to change this so that the application can define what the right behaviour is for generating ID sequences, where you might want to, for example, use the request path as part of the ID sequence generation.