Fix restoring BE panels search functionality

This fix restores the BE search functionality. It is mainly a backport of the Contao3 branch.

Targets and hopefully resolves following bugs and issues:

https://github.com/MetaModels/DC_General/issues/74
https://github.com/MetaModels/core/issues/235

It looks like the part with "tl_filters123" is obsolete and never being executed.
The 'try-catch' block in there throws an exception when executed.