columnview: fix listbase cast
ColumnView doesn't inherit from ListBase but from Widget, so the GTK_LIST_BASE (self)
cast may crash.
Pass the correct parameter to ListBase API.
This fixes a regression introduced by !7240 (merged) , I observed a segfault when starting the GTK inspector from Ctrl+Shift+I, here a backtrace from gtk4-widget-factory: bt_widgetfactory_columnview.txt
Edited by gwillems