Use shared instance of the process list

This is just David's patch rebased. Differences are that we can manage the static variable as function local one because it's a bit simpler in my opinion and we still need to pass the attributes vector to the attributes model because the cgroup model adds additional ones.

Use shared instance of the process list

We might want to present the process list in several different views.
The process list is quite expensive, so if we ref-count the core we can
use it in multiple places.

Timers remain in their respecitive views as Processes has it's own
internal throttling mechanism.

Because that leaves us with a ref-counting issue on which plugin
attributes are enabled or not, that is changed to be implicit based on
whether we have dataChanged connected or not; which actually simplifies
some code.

Co-authored-by: David Redondo <kde@david-redondo.de>