In preparation for moving the PE side DND code to win32u/user32 and factoring it between drivers.
-- v2: winex11: Allocate the XDND data object dynamically. winex11: Use IDataObject to get CF_HDROP format for WM_DROPFILES. winex11: Cleanup variable names in X11DRV_XDND_SendDropFiles. winex11: Use the IDataObject interface to check for CF_HDROP format. winex11: Cleanup XDND IDataObject methods and variables. winex11: Move the XDND IDataObject implementation around.