Initial submit for the guide explorer feature.

[JaschaW]

Description of Changes

This PR introduces the Guide Explorer, a new UI tool for inspecting and managing Shifter guides.
It provides a structured tree view of all guide components, a details panel for viewing and editing component settings, and supporting utilities for querying and interacting with guide data.

New modules under mgear/shifter/guide_explorer/ include the main widget, tree widget, tree items, data models, and helper utilities.
Existing Shifter modules were updated to integrate the Guide Explorer into the workflow.

Note: This PR depends on the changes in #584 and should be merged after that one.

Testing Done

• Loaded various guides to verify the tree populates correctly.
• Confirmed selection sync between UI and Maya scene (component selection, root selection, empty states).
• Tested settings panel updates when switching between components.
• Reopened the tool across scene reloads to confirm stability and no script errors.

Related Issue(s)

New feature. #591