Add addon_loader.py implementing a six-stage pipeline: scan mods/ for
package.xml files, parse standard fields and optional <kindred>
extensions via ElementTree, validate version compatibility, resolve
load order via graphlib.TopologicalSorter (with legacy fallback),
exec() Init.py/InitGui.py, and populate a runtime AddonRegistry
exposed as FreeCAD.KindredAddons.
Replace hard-coded addon lists in Init.py and InitGui.py with calls
to addon_loader.load_addons(). All QTimer-based Silo integration code
in InitGui.py is unchanged.
Backward compatible: addons without <kindred> elements load with no
constraints using the existing ztools-then-silo order.
Closes#248
