This way, the UI does not need to listen to `DOMContentLoaded` in order
to discover the `#appContainer` element. This allows a lot of the UI
boot process to become synchronous.

Functionally, it stays the same, but the boot process is no longer a
Redux Saga. It is instead an asynchronous function that runs when the
`DOMContentLoaded` event was fired.

Also, the various boot-related sub-processes (initializing plugins,
loading translations, etc...) have been wrapped in dedicated functions.

As it is no longer needed.

As it is no longer needed.

...rather than inlineing it. `JSON.parse` is somewhat more efficient,
because for JSON only a subset of ECMAScript has to be parsed.

The error message has been adjusted to the look of the global
ErrorBoundary, re-using its stylesheet.

This attributes has no references throughout the entire code base. It
appears to be a leftover from some abandoned feature-work.

…totype

The `Neos.Neos.Ui:Application` prototype is designed to:

1. Functionally replace the `Backend/Index.html` Fluid template
2. Act as a basis for future multi-application support

As of this commit, the API of the `Neos.Neos.Ui:Application` is not
final.

...as it is no longer needed.

The fusion path is now dynamically determined by controller / action
This enables additional actions with separate templates for that controller.

…apping the UI

The `ApplicationView` builds the foundational HTML document (via string
concatenation) for bootstrapping the Neos UI.

Formerly, Fusion was used to retrieve and render the initial data for
the UI. This task has been delegated to a set of dedicated classes,
namely:

- `ConfigurationProvider` - retrieves the `nodeTree` and `structureTree`
segments from `Neos.Neos.userInterface.navigateComponent` settings,
`allowedTargetWorkspaces` from the ContentRepository's
`WorkspaceService` as well as the `nodeTypeSchema` and `translations`
endpoints.
- `RoutesProviderInterface` - retrieves all other routes/endpoints
required for communication with the Neos server application
- `FrontendConfigurationProvider` - reads and preprocesses the
`Neos.Neos.Ui.frontendConfiguration` settings that are mostly used by
third-party plugins to share data between server and client
- `NodeTypeGroupsAndRolesProviderInterface` - retrieves information about
node type roles and node type groups. Roles are a dedicated UI-concept that
is meant to distinguish between document, content and collection nodes.
Groups refer the grouping of node types in the creation dialog.
- `MenuProvider` - retrieves all data needed to render the main burger
menu located in the top left corner of the UI.
- `InitialStateProvider` - reads and preprocesses the
`Neos.Neos.Ui.initialState` settings that are used to hydrate
the UI's redux store

The responsibilities of these classes is entirely derived from their
former Fusion counterparts. All of them have been marked `@internal` to
allow for later removal. Nonetheless, it remains possible to implement
their accompanying interfaces and replace their implementations via
`Objects.yaml`. This enables rare edge cases and unplanned
extensibility.

The splash screen, formerly a Fluid template, is now hard-coded into the
`ApplicationView` class, which renders the `Neos.Neos.Ui.splashScreen`
setting obsolete. It has therefore been removed.

With the new `ApplicationView` this code is no longer in use anywhere.

This action used to be fired before all asynchronous operations at the
beginning of the boot process.

Since there's not a single subscriber within the UI code base that
listens to that action, it has now been removed.

This also removes handling of the legacy first level 'defer' setting in
the `Neos.Neos.Ui.resources` settings. If it is set, it will be ignored.

…om is initialized yet

…rything into `initialData`

Previously this feature neos/flow-development-collection@b944521 would cause also the rendering of the neos ui `settings`