Class BackendTemplateView has been a temporary solution
to simplify the implementation of the "TsConfig template
override" feature into smaller patches.

With ModuleTemplate and general view works being mostly
done in the backend, BackendTemplateView should be
removed again.

The patch removes all usages switching nearly all views
to BackendViewFactory, which removes the explicit
dependency to Typo3Fluid ViewInterface since the
EXT:core ViewInterface is not directly bound to Fluid
anymore.

Using BackendViewFactory has the additional advantage
that templates which don't use ModuleTemplate because
they're hook usages or some other "sub-view", can now
be overridden with TsConfig, too.

Using BackendViewFactory also makes the dependency to
ServerRequestInterface explicit. The patch thus hands
$request around at some more places where $request was
only an indirect dependency before.

One special @todo area is FormEngine, which is unable
to have dependency injection (fo...