[TASK] Huge performance improvement in content type selector

...and thus also in editing tt_content records in general. Before, no caching was truly possible. Now, the page TS config file containing all Fluid Content elements is reused when rendering the special "Fluid content type" selector that's usually filled by 1) reading TypoScript and 2) reading individual template files to retrieve configuration.

Thousands upon thousands of function executions will be completely prevented hereafter.