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?).

include/wx/process.h

@@ -77,6 +77,12 @@ public:
     // before the process it started terminates
     void Detach();
 
+    // Activates a GUI process by bringing its (main) window to the front.
+    //
+    // Currently only implemented in wxMSW, simply returns false under the
+    // other platforms.
+    bool Activate() const;
+
 #if wxUSE_STREAMS
     // Pipe handling
     wxInputStream *GetInputStream() const { return m_inputStream; }