[TASK] Streamline ext_localconf.php / ext_tables.php of sysexts

In order to have a proper "way to do things" for extension
developers to look up registration process the following
parts were streamlined:

- Discouraged usage of the pseudo-global variable $_EXTKEY inside
  ext_localconf.php and ext_tables.php in favor of strings.
  For module registrations this part was left out for now as
  module registration will be streamlined separately.
- Use PHP5.5-style ::class in all occurrences instead of escaped
  strings for class names.
- Use $GLOBALS['TYPO3_CONF_VARS'] instead of $TYPO3_CONF_VARS.

Releases: master
Resolves: #65347
Change-Id: Iad03e42d38d65ae1483ef8190f755430ec617e45
Reviewed-on: http://review.typo3.org/37240


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>