Add e_mail_session_get_junk_filter_by_name().

Looks up an EMailJunkFilter extension by its filter name, as specified
in its class structure.
EMailJunkOptions: Bind to "junk-filter" instead of "junk-filter-name".

Use transformation functions to convert between EMailJunkFilter and the
junk filter name.
ESettingsMailSession: Bind to "junk-default-plugin".

Use mapping functions to convert between the filter name string and
the corresponding EMailJunkFilter extension.
EMailSession: Remove "junk-filter-name" property.

No longer needed.  Following up on a pre-GSettings "FIXME" item.