[FEATURE] Raw fluid template page type

This adds a new page type (doktype=187) which when used, displays a field to enter a direct Fluid template reference. If reference is in EXT:key/path.html format a Layout selector will read possible Layouts from that extension's view configuration as configured by TypoScript on the currently active page's root line. If no extension key could be detected this way, the key `fluidpages` will be used as fallback (which means view paths will be taken from `plugin.tx_fluidpages.view` instead of `plugin.tx_$resolvedKey.view`).

Switching the selected Layout is currently not implemented.