variable_watch: Don't share memory for callbacks

The command itself is owned by the cmMakefile class, but the
cmVariableWatch which holds a pointer to the cmVariableWatchCommand via
the client_data for the callback outlives the cmMakefile class in the Qt
GUI. This means that when the cmMakefile is destroyed, the variable
watch is still in effect, but with a stale pointer.

To fix this, each callback is now a separate entity completely and
doesn't rely on the command which spawned it at all.

An example CMakeLists.txt which demonstrates the issue (only displayed
in cmake-gui, so no tests can be written for it):

    set(var 0)
    variable_watch(var)

Source/cmVariableWatchCommand.h

@@ -14,13 +14,6 @@
 
 #include "cmCommand.h"
 
-class cmVariableWatchCommandHandler
-{
-public:
-  typedef std::vector<std::string> VectorOfCommands;
-  VectorOfCommands Commands;
-};
-
 /** \class cmVariableWatchCommand
  * \brief Watch when the variable changes and invoke command
  *
@@ -39,6 +32,9 @@ public:
   //! Default constructor
   cmVariableWatchCommand();
 
+  //! Destructor.
+  ~cmVariableWatchCommand();
+
   /**
    * This is called when the command is first encountered in
    * the CMakeLists.txt file.
@@ -83,13 +79,8 @@ public:
 
   cmTypeMacro(cmVariableWatchCommand, cmCommand);
 
-  void VariableAccessed(const std::string& variable, int access_type,
-    const char* newValue, const cmMakefile* mf);
-
 protected:
-  std::map<std::string, cmVariableWatchCommandHandler> Handlers;
-
-  bool InCallback;
+  std::set<std::string> WatchedVariables;
 };