Use correct column type when adding columns to wxDataViewListCtrl.

Using base class methods such as AppendBitmapColumn() resulted in
wxDataViewListCtrl::AppendColumn() being called but this function always
assumed the column was of "string" variant type -- which was, of course, false
for bitmap columns and so resulted in heap corruption (thanks to the
wonderfully type unsafe code using wxVariant) and a crash.

Get the correct type to use from the column itself now to fix this.

Closes #16008.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75946 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775