UXKit 0.74 — the UI toolkit for xc, as an installed library, for xcc 0.74

Put 3p/ beside the versioned xcc install, so that /opt/xcc/3p/uxkit/ sits next to
/opt/xcc/0.74/:

    sudo cp -R 3p /opt/xcc/

A program then names UXKit once and builds with nothing else:

    #use <UXKit>
    ...
    UXApplication* app = new UXApplication();
    app.setDriver(UXPlatform.driver());     // the driver for the target it is built for

    xcc -A arm64  app.xc -o app            # macOS
    xcc -A win64  app.xc -o app.exe        # Windows: ship libUXKit.dll beside it
    xcc -A x86_64 app.xc -o app            # Linux: ship libUXKit.so and libUXGtk.so beside it;
                                            # the machine needs GTK 4
    xcc -A wasm32 app.xc -o app            # the web: ship libUXKit.wasm, libUXKit.json,
                                            # ux_web_page.js and ux_web_browser.js beside it

A program may check the library's version with UXAbi.xc (3p/uxkit/xc/).

UXKit is LGPLv3 (COPYING.LESSER, which builds on COPYING). Its source is in src/;
install.sh there rebuilds and installs the libraries.
