Highlight all Slint code and don't do the preview on snippets where it
doesn't make sense.
Some snippets, such as the bare statement or expression snippets, are
now highlighted, but they are continued to be excluded from the doctest.
Don't fix the canvas to 100/100 physical pixels, as not all examples fit into that. Instead
let the GL backend resize according to constraints,
and add some missing sizes to some examples.
Only the last shown preview was interactive. This was because we started the winit event loop for each preview,
which kind of invalidates the previous instance in winit.
Instead, create all components, call show() on each instance to register the window with winit
and spin the event loop with its exception magic only once.
