Add qsb inspector tool as a manual test

qsb_inspect.py is a pure Python .qsb inspector, a TUI alternative
to qsb -d that decodes and shows some of the metadata in a more
friendly manner than the dump, while allowing rapid keyboard
navigation. Needs Textual. (pip install textual)

Usage:
python qsb_inspect.py <file.qsb>
python qsb_inspect.py <directory>
python qsb_inspect.py <git commit SHA>
python qsb_inspect.py <Gerrit change ID>

SPIR-V disassembly or converting to GLSL via SPIRV-Cross is
provided by launching external tools that ship with the Vulkan
SDK.

This is meant as a purely development-focused tool allowing to
rapidly inspect the contents of one or more .qsb files, no matter
where they come from, from a Qt build under the .qsb directory
generated by CMake, from a git commit, or, most importantly,
straight from a change posted on Gerrit, without depending
on a Qt build and the QShader deserialization infrastructure
at all, thus allowing independent verification of the .qsb
files' contents.

Task-number: QTBUG-146998
Change-Id: Ie1b6b808d4ddae088046ef2257be985cee012edd
Reviewed-by: Andy Nichols <andy.nichols@qt.io>