Add wxVector(std::initializer_list<U> list) ctor (3.2)

[vslavik]

(This is 3.2-only as this code is gone on master. I think it doesn't have ABI implications and so is safe to add to 3.2, and that it is worth having, because Linux distros ship w/o wxUSE_STD_CONTAINERS, but I realize your mileage may vary, so feel free to reject it.)

This is analogous to code present for wxArray and makes using wxVector easier. As a motivating example, adding data to wxDataViewListCtrl is a bit of a PITA w/o it:

wxVector<wxVariant> data;
data.push_back("foo");
data.push_back("bar");
data.push_back("zar");
list->AppendItem(data);

vs.

list->AppendItem({"foo", "bar", "zar"});

[vadz]

Looks good to me, thanks! Will push soon.