New feed implementation in \PageRegular

Adds a new Hook `getPageStyleSheets` to extend the default header style output.
It is not possbile to register **custom module feeds** (or custom link-rel-types) on the fly.

This **additional hook** aims to the Frontend output of a regular Page and cleans up the `\PageRegular` class. 

The default feeds (`newsfeeds` & `calendarfeeds`) are **hardcoed** in `\PageRegular`. The functions have been moved to their module configuration files and classes by using the new hook.


Changes:

1. **Module News**

  * Registered a new hook `getPageStyleSheets` 
  * Added a new function to `\News` to add the news feeds (`getPageStyleSheets` )

2. **Module Calendar**

  * Registered a new hook `getPageStyleSheets` (config)
  * Added a new function to `\Calendar` to add the calendar feeds (`getPageStyleSheets`)


As a further step, I will try to create a better feed registration in `tl_layout`.