Command completion with tab key

[MaxGyver83]

In the command prompt, press <tab> to get a list of all available commands and pick one (using vis-menu). This works also after typing the first letters of a command (p.e. :la<tab>).

Implements #1167 (and #100 more or less).

I'm aware that @martanne wants to avoid this feature. But I want to share it anyway for those who want to apply this patch to their local vis build.

[mcepl]

See also https://lists.sr.ht/~martanne/devel/%[email protected]%3E for a discussion how to remove a warning caused by this patch.