The call to gdk_win32_clipboard_request_contentformats() can return NULL even
without an error condition being hit (such as when the system clipboard is
empty), so check whether the returned GdkContentFormat pointer is not NULL
before calling gdk_clipboard_claim_remote(), which expects it to be not NULL,
otherwise we face a warning from that funtion and the subsequent
g_object_unref().
This at least partially fixes issue #4796.
Rename GdkWin32Selection to GdkWin32Clipdrop, since GdkSelection
is mostly gone, and the word "selection" does not reflect the
functionality of this object too well.
Clipboard is now handled by a separate thread, most of the code for
it now lives in gdkclipdrop-win32.c, gdkclipboard-win32.c just uses
clipdrop as a backend.
The DnD source part is also put into a thread.
The DnD target part does not spin the main loop, it just
emits a GDK event and returns a default value if it doesn't get a reply
by the time the event is processed.
Both clipboard and DnD use a new GOutputStream subclass to get data
from GTK and put it into a HGLOBAL.
GdkWin32DragContext is split into GdkWin32DragContext and GdkWin32DropContext,
anticipating a similar change that slated to happen to GdkDragContext.
OLE2 DnD protocol is now used by default, set GDK_WIN32_OLE2_DND envvar to 0
to make GDK use the old LOCAL and DROPFILES protocols.
https://bugzilla.gnome.org/show_bug.cgi?id=773299
