Rework recording list

[pbiering]

This fixes issue https://github.com/vdr-projects/vdradmin-am/issues/13

by huge optimization of the related record list code

• retrieve list from VDR and create folder structure hereby already (incl. count and length)
• filter records by selected "parent"
• remove now obsolete (anyhow not recommended) caching code and other no longer leftovers