Add Selection by prim type for Composer UI

[chunghob]

Sync with @mati-nvidia about adding more ways to get prim selection by type to for better discoverability.

import omni.usd

stage = omni.usd.get_context().get_stage()
selection = omni.usd.get_context().get_selection()

prim_paths = []
for prim in stage.Traverse():
    # Change string to desired type i.e. Xform, Mesh, Scope, etc
    if prim.GetTypeName() == "Material":
        prim_path = prim.GetPath()
        prim_paths.append(str(prim_path))

print(prim_paths)
selection.set_selected_prim_paths(prim_paths, True)