Inv(#1020): add code to monitor clean-up by GTK

I am still suspicious the cleanup mechanism for child widgets works as expected...
But right now, we can't verify that, since on shutdown we get an assertion failure
from ld.so "dl-close.c: 762: _dl_close: Assertion `map->l_init_called' failed!"
Seems we're loading the GUI plugin not properly

src/gui/panel/timeline-panel.cpp

@@ -127,6 +127,8 @@ namespace panel {
   
   namespace {
     _Fmt childID("Chld-%02d");
+    
+    int instanceCnt = 0;
   }
   
   
@@ -138,9 +140,37 @@ namespace panel {
     {
       signal_clicked().connect(
         mem_fun(*this, &ChildEx::onClicked));
+      
+      ++instanceCnt;
     }
   
   
+  ////////////////////////////////////////////////////////////////////TICKET #1020 : verification code for instance management
+  ChildEx::~ChildEx()
+    {
+      --instanceCnt;
+      if (instanceCnt > 0)
+        cout << "  💀  still "<<instanceCnt<<" children to kill...";
+      else
+      if (instanceCnt == 0)
+        cout << "+++ Success: all children are dead..."<<endl;
+      else
+        cout << "### ALARM ###"<<endl
+             << "instanceCnt == "<<instanceCnt <<endl;
+    }
+  
+  void
+  __verifyDeadChildren()
+    {
+      if (instanceCnt == 0)
+        cout << "+++ Success: all children are dead..."<<endl;
+      else
+        cout << "### ALARM ###"<<endl
+             << "instanceCnt == "<<instanceCnt <<endl;
+    }
+  ////////////////////////////////////////////////////////////////////TICKET #1020 : verification code for instance management
+  
+  
   void
   ChildEx::onClicked()
   {

src/gui/panel/timeline-panel.hpp

@@ -56,11 +56,15 @@ namespace model{
       
     public:
       ChildEx();
+     ~ChildEx();
       
     private:
       void onClicked();
     };
   
+  void __verifyDeadChildren();
+  
+  
   
   /**
    * Dockable panel to hold timeline widget(s).