Enhancement: Skip pseudoroot prim in Prim Hierarchy + fix Prim rename in variant edit content + some optimizations

[BigRoy]

Changes:

• Set PrimHierarchyModel root to / so that pseudoroot is not shown but show direct children instead + small optimization
• Optimize filter + fix typo in class name
• Avoid crash on renaming variant, see: https://github.com/PixarAnimationStudios/OpenUSD/issues/2844
• Prim Spec Editor: Disallow editing the pseudoroot
• Fix skipping variants not found