Add a new PointCloudColorHandler for "label" field

This pull request adds a new color handler specialized for the "label" field. It assigns colors to points based on the labels. The colors are taken from Glasbey lookup table to ensure max distinctiveness.

This addition allows an end user to easily visualize labeled point clouds by simply writing:

```cpp
pcl::PointCloud<pcl::PointXYZL>::Ptr cloud (new pcl::PointCloud<pcl::PointXYZL>);
pcl::visualization::PCLVisualizer viewer;
viewer.addPointCloud (cloud);
```

More specifically, this pull request:
  * Moves Glasbey lookup table from 'point_cloud_image_extractors.hpp' to `pcl_common`
    (because now it is needed in both `pcl_io` and `pcl_visualization`);
  * Updates the lookup table with nicer colors;
  * Adds `PointCloudColorHandlerLabelField` class;
  * Updates `pcl_viewer` app to use the new handler when "label" field is present;
  * Updates `PCLVisualizer` to provide convenience specializations for labeled points (`PointXYZL`).