Enhancement: Introduces PagerInterface::isDeterministic (refs #8164)

[mvhirsch]

Subject

This is my suggestion to indicate an enduser that the list doesn't know how many elements ("all elements") will be affected by now. I decided to use a universal + sign to indicate it.

Default behavior:

Behavior SimplePager:

I'm not sure if I should add this to abstract Pager class without finalizing it. I rather suggest to implement abstract function isDeterministic and force any implementation of Pager to set it. WDYT? As this is not BC, I didn't choose this path (any custom implementation might be broken after a patch/minor update).

This refs #8164 but does not finally closes it. I'll update the way SimplePager count results using thresholds in a separate MR. Together they will solve #8164.

I am targeting this branch, because it's BC.

Refs #8164

Changelog

### Added
- Introduces `PagerInterface::isDeterministic` to render yet unknown result count in `SimplePager` properly