PCLVisualizer: save and restore camera information

Press ctrl + s to save camera parameters and ctrl + r to restore the
saved camera parameters.

If a camera file is specified with "-cam" option or pcd file(s) is(are)
available in the command line, the camera parameters will be written to
the corresponding file and can be restored later from that file.
In this case, PCLVisualizer will restore camera parameters automatically
from the corresponding file when the pragram restarts with the same
command line input.
I'm pretty sure it's used in the very next line (4382). Besides, the standard (at least in PCL) way to prevent unused parameter warning is to remove the parameter name leaving only its type, i.e.:

```cpp
std::string
pcl::visualization::PCLVisualizer::getUniqueCameraFile (int, char **argv)
{
  ...
}
```