[BUGFIX] Category localization cannot be rendered

Localized sys_category records cannot be rendered in fronted,
since only records with a positive pid value are overlaid -
however, sys_category is stored on the root-level.

Resolves: #56059
Releases: 6.2, 6.1
Change-Id: I211dad5086adf30723ef4f44757fe6a3121ac29e
[FEATURE] Introduce MM_oppositeUsage property

On copying a sys_category record, a new MM reference is created,
however without setting the "fieldname". This value is basically
defined from the opposite entity with MM_match_fields, but
cannot be accessed.

This change introduces the MM_oppositeUsage property for MM
definitions in the TCA.

Example:
'config' => array(
  'allowed' => '*',
  'MM' => 'tx_myextension_first_second_mm',
  'MM_oppositeUsage' => array(
    'tt_content' => array('somefield'),
    'tx_myextension_domain_model' => array('some_property'),
  ),
),

Resolves: #56061
Releases: 6.2
Change-Id: I26fd54688cf44d6a026ccdb3ca9276583b32acd3