DropDown widget

Hi,

I have wrapped the new GtkDropDown widget from GTK 4. I don't know if
someone is already working on that, I mostly did it because I wanted to
learn how gtkmm wrapping works. If you're interested, I have created a
pull request in gnome gitlab.

The class is not complete, but it's enough to get a drop down with
strings in it working. I wrapped the following classes: DropDown,
ListItem, ListItemFactory, SignalListItemFactory and StringList. What's
left is the Expression class, something I felt wasn't suitable for a
gtkmm wrapper beginner. :)

I have some class documentation left to do, that's why I added "WIP" to
the pull request. But I would be very thankful if someone would review
the pull request. I guessed a lot while working on it.

https://gitlab.gnome.org/GNOME/gtkmm/-/merge_requests/29/diffs

/Andreas