multiple isolated pages within one bootstrapping instance

[x1B]

We'd like to add a new API to instantiate individual pages without having to load a flow (but still allowing to run a flow side-by-side).

The new API should be available as laxar.bootstrap( ... ).page( ref, domContainer ), and support chaining of .flow(), .bootstrap() or other .page() calls.

To allow for this feature, we need to do (at least) the following:

• make sure IDs (widgets / event bus subscribers, axId) are generated without conflict if e.g. the same page is instantiated twice,
• isolate begin/endLifecycle events (and probably also the initial didChangeLocale) across pages, by having a dedicated "event bus view" for each page, which uses some kind of "source token" with each event to determine its isolation context,
• for this, we need to mangle the bootstrapping item IDs into the generated widget IDs.