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?).

interface/wx/process.h

@@ -80,6 +80,20 @@ public:
     */
     virtual ~wxProcess();
 
+    /**
+        Activates a GUI process by bringing up its main window to the front.
+
+        This is a convenient method which tries to bring this process to the
+        users attention.
+
+        Currently this is implemented in wxMSW only and simply returns @false
+        under the other platforms. Notice that this function can also return
+        @false under MSW if, for example, the process doesn't have any windows.
+
+        @since 3.1.0
+     */
+    bool Activate() const;
+
     /**
         Closes the output stream (the one connected to the stdin of the child
         process).