The change in PopupMenu::ImplExecute is needed because
MenuBarManager dispatches commands synchronously, and a
command can try to dispose the parent window of the
MenuFloatingWindow (e.g. right clicking inside the report
header, and choosing "Delete Header"), which will lead to
Application::Abort in a debug build. So we must get rid of
the MenuFloatingWindow before handling menu selection.

Change-Id: I1657cd16a005207abc31f4b65595ab78ed235e37
Reviewed-on: https://gerrit.libreoffice.org/34407Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Maxim Monastirsky <momonasmon@gmail.com>