... to already open soffice process from newly spawned one on Windows.

When an application takes user input, a timeout is started during
which other processes cannot create foreground windows that might
steal focus, and thus interrupt user input. The timeout is defined
by SPI_SETFOREGROUNDLOCKTIMEOUT (see SystemParametersInfo) and
ForegroundLockTimeout registry setting (see
https://technet.microsoft.com/en-us/library/cc957208). If an
application that currently doesn't have right to become foreground
tries to show popups in this interval, the popup will stay on
background, and only flash in taskbar.

The application that has the right to steal focus (see the list in
https://msdn.microsoft.com/en-us/library/ms632668) may transfer its
right to another process using AllowSetForegroundWindow function.
So, the intended effect is this:

1. User interacts with some foreground process (e.g., Explorer);
   a timeout is started to prevent non-privileged processes from
   stealing focus;
2. As the result, the process launches a new soffice process, which
   has privilege to create foreground windows (as it is started by
   foreground process);
3. It communicates with already started soffice process, which is
   currently in background, and so doesn't have privilege to create
   foreground windows until timeout expires;
4. It transfers its right to the already started soffice process,
   and then issues the required commands that might lead to need to
   show popup windows.

Change-Id: I4208665c2ae4106fa06e72269f4c3804af40d582
Reviewed-on: https://gerrit.libreoffice.org/48839Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Mike Kaganski <mike.kaganski@collabora.com>