[PREVIEW-WIP] PERF: Improve startup time by 8% with lazy loading of wrapped libraries

Created by: jcfr

*** WORK IN PROGRESS: For now, you have to make sure SlicerApp-real and Slicer launcher are built to ensure the successful generation of the json files ***

Startup time reduced from 3.8s to 3.5s with a "cold cache" and from 2.7s to 2.38s with a "warm cache".

For each logic/mrml/dm/widgets python modules, a json files listing the associated attributes is generated. Then, when the application is initialized, the "slicer" module is created as a "lazy" module with the attributes associated with logic/mrml/dm/widgets set as "not loaded".

Finally, as soon as an attribute not yet loaded is accessed, the specialized __getattribute__ loads the associated python module and update the module dictionary.

The "lazy" module has been adapted from "itkLazy.py"

Results have been gathered on Ubuntu 15.10 on a workstation with the following specs: 64GB / M.2 PCIe NVMe SSD / Quad Core 3.80GHz

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