Class-based property set processors

And small fixes of issues I found while develop:

https://github.com/argnist/revolution/commit/f37f00ad03d9d3b58cc3c729d8f443176ab1868b You could choose element and then choose another element class but old id of element stayed. Fixed it by reset combobox value.

https://github.com/argnist/revolution/commit/79bc3850426a9205ae056950f478f74bd009aecb $modx->logManagerAction() writes "unknown" if item is empty and item can be different string value. But this processor handled only primaryKey value and converted item to it.  So any string item converts to 1. Fixed by additional conditions.