V547 Expression 'procHandle != nullptr' is always false.

The code was nonsensical overall. First, the launched process handle
was never returned by ShellExecuteExW, because SEE_MASK_NOCLOSEPROCESS
wasn't used, so GetProcessId couldn't succeed. Then, nullptr window
handle was passed to GetWindowThreadProcessId, thus never returning a
meaningful result.

This reimplements this to find the launched process' main window by
first waiting for process idle (up to 1-second delay is possible),
then enumerating all the top-level windows and checking their process.

Change-Id: I5fb4c04147b3f9414e27650a023f7844523c18bd
Reviewed-on: https://gerrit.libreoffice.org/62478
Tested-by: Jenkins
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>