make sure that wxProcess always have a valid PID set; add test unit for wxExecute,wxShell,wxProcess based on the code in the console sample; add some more infos/notes in wxProcess docs

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@59784 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/process.cpp

@@ -102,7 +102,13 @@ void wxProcess::OnTerminate(int pid, int status)
 
 void wxProcess::Detach()
 {
-    SetNextHandler(NULL);
+    // we just detach from the next handler of the chain (i.e. our "parent" -- see ctor)
+    // not also from the previous handler like wxEvtHandler::Unlink() would do:
+    
+    if (m_nextHandler)
+        m_nextHandler->SetPreviousHandler(m_previousHandler);
+
+    m_nextHandler = NULL;
 }
 
 // ----------------------------------------------------------------------------