[TASK] Introduce Hooks to create DB fields and keys dynamically

For the purpose of modifying the database schema upon extension
installation and database compare in install tool I implemented hooks
to append sql create statements dynamically.

Hooks are registered via:

$TYPO3_CONF_VARS['SC_OPTIONS']['typo3/mod/tools/em/index.php']
['checkDBupdates'] for changes in the extension manager and has to
implement tx_em_Index_CheckDatabaseUpdatesHook interface

$TYPO3_CONF_VARS['SC_OPTIONS']['ext/install/mod/class.tx_install.php']
['checkTheDatabase'] for changes in the install tool and has to
implement tx_em_Index_CheckDatabaseUpdatesHook interface

Change-Id: I66ff195ae8f656961f5251d7722a16a554aca219
Resolves: #29594
Releases: 4.6
Reviewed-on: http://review.typo3.org/4870
Reviewed-by: Fabien Udriot
Tested-by: Fabien Udriot
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers