Fix duplicate inline filter references not working

[Pablete1234]

Fixes duplicate inline filters complaining about no two filters being able to use the same id, when used as child nodes and not attributes.

  <any id="test-filter-1">
    <filter id="all(round-1, round-1-finished)"/>
    <filter id="all(round-2, round-2-finished)"/>
  </any>

  <all id="test-filter-2">
    <filter id="all(round-1, round-1-finished)"/>
    <filter id="all(round-2, round-2-finished)"/>
  </all>

In this scenario, pgm would complain that all(round-1, round-1-finished) is a duplicate id, because it is assuming it needs to register it, due to not being an XMLFilterReference (it's an all filter). This PR makes it so if a reference xml creation is for an already defined filter, it will use that one instead of creating a new one. That solves both not finding, and also adds caching to re-used inline filters that get used like this.