Must not call getTransferDataFlavors with m_Mutex held;

6  TransferableHelper::getTransferDataFlavors() (this=0x567b980) at svtools/source/misc/transfer.cxx:392
7  VclGtkClipboard::setContents(com::sun::star::uno::Reference<com::sun::star::datatransfer::XTransferable> const&, com::sun::star::uno::Reference<com::sun::star::datatransfer::clipboard::XClipboardOwner> const&) (this=0x3199550, xTrans=uno::Reference to (SwTransferable *) 0x567b9a8, xClipboardOwner=uno::Reference to (SwTransferable *) 0x567b9b0) at vcl/unx/gtk3_kde5/../gtk3/gtk3gtkinst.cxx:618
8  TransferableHelper::CopyToSelection(vcl::Window*) const (this=0x567b980, pWindow=0x301b0c0) at svtools/source/misc/transfer.cxx:960

CopyToSelection() uses SolarMutexReleaser, then getTransferDataFlavors()
has a SolarMutexGuard; this will deadlock against the main thread
that holds SolarMutex and calls VclGtkClipboard::setContents() itself,
from VclGtkClipboard::OwnerPossiblyChanged().

Change-Id: Ibda0b6bce6f3388c45cee8077dd977abb3dda366
Reviewed-on: https://gerrit.libreoffice.org/67121
Tested-by: Jenkins
Reviewed-by: Michael Stahl <Michael.Stahl@cib.de>