[processing]correctly update toolbox after editing/creating/deleting models and scripts
[processing] refactoring to put all algorithm providers under a single module
[processing] improvements in help for algorithms

Avoid creating temporary helpfiles
Added basic converter to convert rst into html on the fly
Added SAGA help files
[processing] corrected wrong name of script help file
[processing] added grass7 files (forgot to add them on refactoring)
[processing] fixed indentation issues
[processing] prevent grass7 config parameters from shadowing grass6 parameters
[processing] fixed wrong help url to Processing manual
[processing] prevent grass installation check returning None when folder not set under windows
[processing] fixed wrong path to grass icon in grass7 provider
[processing] recover changes introduced in b928d165206a0994e593903d149c875131bfd6c6, lost on refactoring and merging