Add wxProcess::Activate() and implement it for MSW.

When launching child processes it can be convenient to be able to switch to
them later, provide a method in wxProcess to do it.

Currently this is only implemented in wxMSW but could almost certainly be done
for wxOSX too (it can be done using Apple Script, so presumably there is a way
to do it programmatically as well) and could be also made to work at least
under some Unix systems by emulating what wmctrl does (or just launching it?).

docs/changes.txt

@@ -69,6 +69,7 @@ All (GUI):
 - Add wxAddRemoveCtrl.
 - Add wxAppProgressIndicator for MSW (Chaobin Zhang) and OS X (Tobias Taschner).
 - Add wxEVT_MAGNIFY mouse event (Joost Nieuwenhuijse).
+- Add wxProcess::Activate().
 - Make results of wxDC::DrawEllipticArc() consistent across all platforms.
 - XRC handler for wxAuiToolBar added (Kinaou Hervé, David Hart).
 - Add wxCursor::GetHotSpot().