fixed addTableAttribute for class attribute

Moved the setting for the default table class to rex_list(), because the definition in get() would override settings by addTableAttribute.
Conflicts:
	redaxo/include/addons/image_manager
	redaxo/include/addons/textile
fir for fixed addTableAttribute for class attribute

early setting of class attribute won't work. Now check for a existing class attribute in rex_list::get().
Conflicts:
	redaxo/include/addons/image_manager
	redaxo/include/addons/textile