Adding helper class to launch qtassistan from ApplicationWindow.

Re #6630. The helper class is rhobust, but only tested on linux so
far. Still needs help with other operating systems and working on
installed versions.
Re #6630. More logging and os-dependant assistant location.
Re #6630. qcollectiongenerator is now required by cmake.
Re #6630. Beefed up location of qt assistant.
Re #6630. Added qtassistant to all target.
Re #6630. Move qtassistant results to better serve windoze.
Re #6630. qhc file move completed.
Re #6630. Checkpointing refactor of qtassistant file reorg.
Re #6630. More shuffling about of directories for qtassistant.
Re #6630. Changed urls for opening help pages from MantidPlot.
Re #6630. Finished moving files around.
Re #6630. Moving from lxml to minidom.
Re #6630. Refactor minidom stuff.
Definition of the Interface of ScriptRepository.h

re #6175
Implementation of ScriptRepositoryImpl

Implementation of the constructor, connect, install, download, info, listFiles, fileStatus methods.

re #6175
Introduction of the python script for ScriptRepository Web


It updates the web service repository.json file

re #6175
Re #6630. More on the move from lxml to minidom.

Had to reorder parameters in the helper functions to make life a bit easier.
Re #6630. Moved algorithm_help.py to use minidom.
Re #6630. More code moving to minidom.
Re #6630. Moving the last bit of lxml to minidom.