Implement event mediator

[rozyczko]

Description

Created EventMediator Infrastructure for FittingWidget to modularize and simplify event handling between multiple widgets and sections.

The Event Mediator (sometimes called Event Aggregator or Publish-Subscribe with a central mediator) is a behavioral design pattern that allows objects (publishers) to emit named events with optional data, and other objects (subscribers/listeners) to register interest in those events — all without the publishers and subscribers knowing about each other directly.

At its core, EventMediator is a glorified dictionary whose keys are event names (or types) and whose values are lists of callables (the “slots”). Everything else (subscribe/unsubscribe/publish, error handling, weak references, async support, sender filtering, signal objects, etc.) is just sophisticated dressing on top of that one dictionary.

How Has This Been Tested?

Local tests + unit tests created

Review Checklist:

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Documentation (check at least one)

• [ X] There is nothing that needs documenting
• Documentation changes are in this PR
• There is an issue open for the documentation (link?)

Installers

• There is a chance this will affect the installers, if so
  • Windows installer (GH artifact) has been tested (installed and worked)
  • MacOSX installer (GH artifact) has been tested (installed and worked)
  • Wheels installer (GH artifact) has been tested (installed and worked)

Licensing (untick if necessary)

• The introduced changes comply with SasView license (BSD 3-Clause)