Implemented GTK4 widget and gtk4 demo

[sp1ritCS]

unlike the other platforms, this just creates a Widget that itself uses the cairo platform to render itself.

The Widget itself doesn't expose anything from microtex, and exclusively exports a C-API, making it compatible with any language that has support for GObject-introspection (python, javascript, rust, nim, java, ...).

On meson, it also generates docs using gi-docgen (assuming -DTARGET_DOCUMENTATION=true is set).