Merge pull request #74 from jdlx/core/rex_select/optgroup_backport

[rex select] optgroup backport (fixes #39)
Conflicts:
	redaxo/include/addons/image_manager
added parameter to subclass

Conflicts:
	redaxo/include/addons/image_manager
Merge pull request #75 from jdlx/core/rex_value_array_to_json

[REX_VALUE] array handling (fixes #66)
Conflicts:
	redaxo/include/addons/image_manager
	redaxo/include/functions.inc.php