fdo#67011 KDE4: prevent blocking in Display::Yield

SalX11Display registers a Yield handler, which splits check and
and processing into two functions, which both lock the yield mutex.

Normally this no problem, but during a Drag'n'Drop operation the
D'n'D thread also checks and processes XEvents (for D'n'D). So the
XNextEvent in Display::Yield will actually block, if the seen XEvent
was for D'n'D and was already processed.

(cherry picked from commit 6c7374f071d998f726cd4a5b67baf54e357d096b)

Change-Id: Ifb0631b38abc0ddab23cd74aad05120b4b3dacad
Reviewed-on: https://gerrit.libreoffice.org/8710
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>