[PHP MapScript] Need a freeLabelCache() method

[dmorissette]

This came up in an Email to the Mapserver-dev list about clearing the label 
cache after a map draw.

mapscript.i (SWIG) has a labelCacheObj->freeCache() method, and we need to add 
the equivalent to PHP MapScript.  The code of the method in mapscript.i should 
first be moved to a msClearLabelCache() function in libmap.a to avoid 
duplicating code between multiple version of MapScript.

[assefa]

Create a labelcacheObj accessible through the mapobject (map->labelcache).
The only method availabel on theis object is free (map->labelcache->free()).

Note : the function msFreeLabelCache was already moved in mapfile.c