allow declarative configuration of ELContextListener in faces-config

Currently the only way of registering an ELContextListener seems to be programmatically via
Application#addELContextListener().

It would be interesting to allow declarative configuration via a key (e.g. el-context-resolver) in the
section of the faces-config schema.
#### Environment
Operating System: All
Platform: All
#### Affected Versions
[2.0]

Start with Application#addELContextListener() and the faces-config schema, then trace how the section is parsed and applied. Define the declarative key and verify that configured ELContextListeners are registered through the same application path; the issue does not name specific files or tests, so inspect the existing configuration and listener tests first.

Written by the indexing model from the issue text.