Add context menu for layers in the layerTree, with the option to zoom to layer extent

Hi, 

I've added the context menu for layers in the layerTree, with the option to zoom to layer extent.
For now it's enabled only on the leafs of the layerTree, and it allows you to zoom to layer extent.
Very useful if you have many layers and a large map, and you want to see its extent.
It gets the layer extent from the loaded QGIS project file, so:
 - for vectorial/raster layers the extents are the same of QGIS;
 - for WMS layers the extents of the layer are the same of the mapserver MAP file, so you should write the exact extents there.