[GTK4] Implement DropTarget async drop handling

Implement GTK4 DropTarget support using GtkDropTargetAsync with accept,
enter/motion/leave and drop handlers.

Add GTK4 drop data flow integration by matching offered GTypes, reading
drop values asynchronously via gdk_drop_read_value_async/finish, and
finishing drops with the selected action.

Tested with DNDExample.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: akurtakov