[PHP MapScript] Add mapObj::insertLayer and mapObj::removeLayer

[dmorissette]

In bug 759, mapObj::insertLayer and mapObj::removeLayer were added to SWIG
MapScript.

They need to be added to PHP MapScript as well.

[dmorissette]

We've got ms_newLayerObj() already to add layers, and status MS_DELETE to remove
them, so this is low priority. Setting FUTURE target milestone.

[dmorissette]

A question about the missing removeLayer() came up on the -users list today. Here is an exceprt from my reply explaining why we never addressed this ticket:

"... then I started to wonder why we never implemented removeLayer(), and it came back to me that removeLayer() returns a standalone layerObj that's decoupled from the mapObj and can even be reinserted into the map at another location later on with insertLayer(). Without looking into this in any detail, it sounds like adding this may be more involved than just adding a new method since PHP MapScript doesn't allow decoupling objects like this at the moment. That and the fact that there are simpler alternatives to do the same thing (i.e. setting status MS_DELETE) probably explains why we never did it."