rich text: add an open-url function

[expenses]

Not implemented for cpp yet

[ogoffart]

Can there be a test?

I believe this could be also use from the AboutSlint widget.

[expenses]

Can there be a test?

I believe this could be also use from the AboutSlint widget.

I looked into adding a test here: https://github.com/slint-ui/slint/compare/master...expenses:slint:open-url-test?expand=1. It's a bit tricky as open-url is SlintInternal. I'd also need to change the open_url function so that it logs something when testing.