Add wxProcess::SetPriority() to allow setting the priority of child processes.

This uses the same conventions as wxThread::SetPriority() but works on the
entire process.

Closes #14931.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73406 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/changes.txt

@@ -546,6 +546,7 @@ All:
 - Add new wxFSW_EVENT_ATTRIB and wxFSW_EVENT_UNMOUNT flags (David Hart).
 - Add separate read/written bytes counters and per-direction NOWAIT and WAITALL
   flags to wxSocket (Rob Bresalier).
+- Add wxProcess::SetPriority() (Marian Meravy).
 - Add wxDir::Close() method (Silverstorm82).
 - Fix wxDateTime::GetWeekOfYear() for the last week of year (aimo).
 - Fix compilation of wxHash{Map,Set} with g++ 4.7 (Nathan Ridge).