OTB processing update

This PR is a complete rework of OTB integration into Processing :
* it depends on the availability of a new parameter, already proposed in https://github.com/qgis/QGIS/pull/1133
* some OTB applications which are not relevant in Processing are not exposed
* some OTB applications with a too complex interface have been splitted into several unitary Processing modules (example : one segmentation module per segmentation algorithm, same for classification)
* application descriptors are in XML format instead of TXT
* for some output parameters, the default Processing behavior of generating ".file" leads to error at runtime. adapters have been implemented when the application needs ".xml", etc.. (cf https://github.com/CS-SI/QGIS/blob/otb_processing_update/python/plugins/processing/otb/OTBSpecific_XMLLoading.py)
* all the helper files to generate the XML app descriptors are provided in a specific "maintenance" directory. it's only intended use is for developers. this folder is not used at runtime.