[FEATURE] Introduce mixed mode for headless

[twoldanski]

Patch introduces new way for checking if we are in headless mode, you can set not enabled, mixed mode (fluid & headless at the same time), or full headless mode.

Mixed mode depends on Accept header sent when requesting the site.

To configure headless mode, please use site configuration flag, for example: 'headless': 0|1|2 Legacy flag (true|false) is respected, but please migrate to int notation

Where: 0 (old: false) = headless disabled 1 (old: true) = headless fully enabled 2 = headless in mixed mode

In mixed mode also you need to load special "Headless - Mixed mode JSON response" typoscript in order to work properly, default one overrides fluid one.