build: script to run qt-designer

[JeppeKlitgaard]

This adds a shell script to run the Qt-Designer tool, which is conveniently shipped with the pyqt6-tools package.

Since there currently is no Qt 6.6 version available on PyPi (see altendky/pyqt-tools#132), it is best to keep the venv of this tool separate from the rest of the development environment. Currently pyqt6-tools seems to have intermittent maintenance, meaning it will also be likely that it will require a specific Python version to be runnable.

With this in mind, the pyqt6-tools tool is managed separately with uv. It was mentioned in #3787 that uv could possible be added to the build toolchain, so I figured this tiny corner could perhaps be the first spot for it.

I completely understand if addition of uv at this stage is not desirable, in which case I will close this PR.

[dae]

This is sufficiently isolated and unessential that I'm not worried about the uvx usage even if the answer to #3787 ends up being "no". Unfortunately though, it's of limited usefulness at the moment:

• It's broken on Windows until Create symlink to enable bash scripts on Windows MSYS2 #3809
• It won't build on modern Macs, as there are no ARM64 wheels, and uvx fails to install when running under Rosetta

I'd be happy to merge if the former can be addressed (I'll follow up on that ticket later), and the script can print a message and exit when run on a Mac.