[FEATURE] Allow cObject registration during runtime

Instead of having a hard-coded list of available
content Objects with complex hook registration,
any extension can now register its own cObject
by adding or overwriting an AbstractContentObject
in ext_localconf.php via
$GLOBALS['TYPO3_CONF_VARS']['FE']['ContentObjects']

This way it is also possible to move legacy content objects
to other extensions than EXT:frontend.

Resolves: #64386
Releases: master
Change-Id: Iac3e13cb32fa99685c6ff9e284ff8c49b26f9759
Reviewed-on: http://review.typo3.org/36146


Reviewed-by: Helmut Hummel <helmut.hummel@typo3.org>
Tested-by: Helmut Hummel <helmut.hummel@typo3.org>
Reviewed-by: Benjamin Mack <benni@typo3.org>
Tested-by: Benjamin Mack <benni@typo3.org>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>