Add SlotSignal for handle automatic disconnect on Signal or SignalSlot destruction.

Example:

```
Win32WindowManager::Win32WindowManager()
{
   // Register in the process list.
   mOnProcessSignalSlot.setDelegate( this, &Win32WindowManager::_process );
   Process::notify( mOnProcessSignalSlot, PROCESS_INPUT_ORDER );

   // When Signal it's destroyed, all slots are disconected.
   // When a SignalSlot it's destroyed, it's disconected from Signal.
}

```
Fix crash on exit T3D when build with CMake.

Process::_signalProcess is destroyed first, and then Win32WindowManager::~Win32WindowManager() try to remove from deleted Signal causing a crash.

SlotSignal handle this automatically.
Merge pull request #628 from BeamNG/fix_cmake_crash_on_t3d_exit

 Fix crash on exit T3D when build with CMake.