Allow using of all contentelements in own extensions

GlobalContentelements lets you use the real contentelements in your own extension. You have to set the ctable to tl_content which lets you use all core- and per extension installed contentelements.

Example News4ward:
* The DCA of tl_news4ward_article sets the  ctable to tl_content
* If the  _GET-Param equals "do", news4ward sets the ptable of tl_content to tl_news4ward_article
* Now news4ward_article can have all installed contentelements.