Keep search filter in session

This PR solves Issue #14 by saving the filter to the frontend user session. The feature can be enabled in the flexform settings of the news plugin:

when POST-ing a new filter request it saves the "Search" to the frontend user session
when reloading the page (eg. by changing a paginated page) the filter is loaded from the session if available
the news demand is set from the session data if available

(Also I removed dead code that previously was used to set the news demand, before the hook was introduced)

I implemented and tested this for TYPO3 v11 This will need more testing but it works fine for my v11 project.

georgringer / news_filter