Adds a method to get the latest version number to AlgorithmFactory. This
is used by the two script building algorithms to determine if a the
algorithm version needs to be included in the history

Adds a method get a single descriptor in the Algorithm factory, this
avoids us having to get all algorithm descriptors. Each time we want a
descriptor every algorithm object is allocated (900+) then discarded for
each workspace

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re #8120

It provides easy access to the highest version of an algorithm
currently registered.
Refs #9521

The factory takes ownership of the injected instantiator object so is
responsible for deleting it if an exception is going to be thrown and
the algorithm will not end up being subscribed.

Refs #9059

Refs #7263

just on the AlgorithmFactory. Refs #970

This allows the algorithm dock in MantidPlot to update when
the algorithm factory is changed. RegistrationHelper has been moved to
its own class to remove the dependency of Parameter.h on DynamicFactory.

This now doesn't require a user to understand the internal
mangling of the algorithm names in order to unsubscribe/replace
and algorithm in the factory.

More python unit tests and a new instantiator for PythonAlgorithms that means we don't need all that CloneableAlgorithm stuff, we can create a new object as requested. Refs #3614

Adds the option to select which algorithm categories are available into the Mantid->options tab of the preferences window.

Known Bug:  Does not work with nested categories at the moment, but right now the only one is DataHandling, and you'd be a fool to hide that.

Python supports multiple categories, no work really as python does not realy support categories any more.

refs #2128
Python and the mantidplot UI respects not showing algorithms if all of it's categories are hidden 

tests:
                  Normally  Hidden   What I want
in tree           Yes       No        No
in search box     Yes       No        No
typed in box      Yes       Yes       Ideally, but not too fussed
mtdHelp           Yes       No        No
intellisense      Yes       Yes       No, but not too fussed
typed in script   Yes       Yes       Yes

Multiple categories are now correctly displayed in Mantidplot.
Hiding of categories is implemented for Mantidplot, but no GUI to select yet.
hidden categories can be set in the properties files with algorithms.categories.hidden

Cleaning up the algorithm version number handling in Python now that we have a new mantidsimple. The existence of a version 1 is no longer a requirement. Re #1591