Keep search filter in session

[cepheiVV]

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.

[kitzberger]

@cepheiVV, TCEforms isn't "compatible" (?) with TYPO3 12 that's why the other options in this file don't have it.