node/Tasks: merge improvements from PBS and make it more generic

node/Tasks: merge improvements from PBS and make it more generic

this copies most of the task grid from pbs, but adds handling so that
users can add aribtrary filter fields

the filter fields always present are:
* since
* until
* task type
* task status

other filters fields can be added by giving an 'extraFilter' array
which must contain widget definitions that emit a 'change' event.
this is then used to update the filters for the api call

also you can add a 'preFilter' object, that sets the filter parameter
only once at the beginning

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>