Bootstrap 5 theme
Currently, Wt has support for Bootstrap 2 and Bootstrap 3 (defaulting to Bootstrap 2 when WBootstrapTheme is used).
We should probably support Bootstrap 5. Bootstrap 4 will not be included since it was in an unfinished state, and the expectation is that users will just use the latest version of Bootstrap.
I think it may also be better to just do WBootstrap2Theme/WBootstrap3Theme/WBootstrap5Theme, since the different versions may simply be too distinct.