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

Re #17600

re #8120

Categories were not being hidden when a parent category was hidden. Now,
categories are hidden whenever either the category itself, or one of its
parents is set as hidden.

Refs #9515

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

Refs #9059

There were a couple of rather major bugs which it's a wonder hadn't been found until now.

The first was that decodeName() could go out the range of the string and return garbage if there wasn't a bar cahracter in the supplied string. A check has been added which will throw an exception if one isn't found.

The second was that getCategories() didn't work at all as the loop wasn't executing due to the variable "it_end" used in the condition not being initialised correctly. It's now initialised correctly to categoryMap.end().

Refs #3344

It was previously not updating the version map when the algorithm was
removed and then subscribing the same thing again would throw an error.
Refs #7263

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.

Fixed problem with configservice not saving new entries.  Also dealt with mantid dock issue on delete