Add MinGW Windows packaging support

[liuliuzo]

Summary

• add configurable MinGW dependency paths for Windows builds while preserving the existing MSVC configuration
• allow an isolated Python 3.8 root and preserve the RESP application version when embedding PyOtherSide
• fix the NSIS registry key and support configurable payload/output paths and optional VC runtime installation
• correct qt.conf so packaged plugins and QML modules are resolved from the application directory

Validation

• built resp.exe with Qt 5.15.2, MinGW 8.1 and Python 3.8
• verified all packaged EXE/DLL/PYD imports have no unresolved non-system DLLs
• launched the portable application for 15 seconds with PATH limited to Windows system directories
• generated the NSIS installer and validated the portable ZIP archive

The generated Windows binaries are local artifacts and are not included in this PR.

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Note

Low Risk
Changes are limited to build and installer configuration; the registry key update may affect alignment with prior installs but does not touch application runtime logic.

Overview
Adds a MinGW (win32-g++) link path in 3rdparty.pri that requires WINDOWS_MINGW_BUILD_ROOT and MINGW_ROOT and pulls static zlib/lz4/zstd/snappy/brotli from those trees, while MSVC win32* behavior stays on the existing NuGet-style libs.

Windows Python embedding can use an optional PYTHON_ROOT instead of a fixed C:\Python38-x64 path. Including PyOtherSide no longer overwrites the app VERSION for PYOTHERSIDE_VERSION; the RESP version is saved and restored around the subproject include.

NSIS packaging fixes the registry key to SOFTWARE\RESP.app, allows SOURCE_DIR / OUTPUT_FILE, and SKIP_VCREDIST for MinGW builds that do not need the MSVC runtime. qt.conf now uses a local prefix with explicit plugins and qml paths so a portable directory layout resolves Qt assets correctly.

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