Add support for custom priority

For some pipelines, it may be interesting to always handle interactions first (picking interactions) or last (camera / low priority interactions).
The current priority system takes into account : widget state > render order > distance to interaction

The new field would be used as the first item in the sorted list of priority.

The issue names no files or tests. Start by locating the current priority sorting and interaction-handling entry points, then trace how widget state, render order, and distance are compared. Done means a configurable custom priority is the first sort key, with the existing ordering preserved for ties.

Written by the indexing model from the issue text.