Allow Plugins to Register Actions in Application Menu

[Copilot]

Removes the menu.json file-based approach for plugin menu registration in favor of a Python API where plugins register callable actions directly. This eliminates the need for any JSON file and enables plugins to attach real Python functions (not just URLs) to menu entries.

Description

Plugins now register menu actions programmatically via a register(plugin) hook or the meshroom.core.registerMenuAction() convenience function. The registered callables are stored with a UUID and invoked through a Qt slot when the user triggers the menu item.

Features list

• Plugin.addMenuAction(label, function, tooltip) — Meshroom-side API for registering a Python callable as a menu entry
• NodePluginManager.executeMenuAction(actionId) — resolves and calls a registered action by UUID
• meshroom.core.registerMenuAction() — convenience wrapper using thread-local context, usable at module level during first import
• register(plugin) hook — called by Meshroom on every plugin load (bypasses Python module caching), recommended pattern
• MeshroomApp.executePluginMenuAction(actionId) QML slot replaces URL-opening logic in the Plugins menu

Implementation remarks

Plugin authoring pattern — define a register(plugin) function in the package's __init__.py:

# myplugin/__init__.py
import webbrowser

def register(plugin):
    plugin.addMenuAction("Open Docs", lambda: webbrowser.open("https://..."), "Opens documentation")
    plugin.addMenuAction("Run Tool", my_tool_function)

Meshroom calls register(plugin) explicitly after each plugin load, so it works correctly even when Python has already cached the module (which would silently skip module-level registerMenuAction() calls on subsequent loads). The registerMenuAction() convenience path is still supported for first-load scenarios.

getAllMenuActions() intentionally excludes the callable from the QML model — only label, tooltip, actionId, and pluginName are exposed. QML calls back into Python via the executePluginMenuAction slot which resolves the callable by UUID.

Original prompt

Add a dynamic entry in the general menu (file, edit, etc), so plugins could add custom elements in the menu dynamically.

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[codecov]

Codecov Report

❌ Patch coverage is 91.81818% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.80%. Comparing base (bf28c8d) to head (3f5032d).
⚠️ Report is 15 commits behind head on develop.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
meshroom/core/__init__.py	61.11%	7 Missing ⚠️
tests/plugins/meshroom/pluginA/__init__.py	75.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3021      +/-   ##
===========================================
+ Coverage    82.35%   82.80%   +0.45%     
===========================================
  Files           69       71       +2     
  Lines         9367     9522     +155     
===========================================
+ Hits          7714     7885     +171     
+ Misses        1653     1637      -16     

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