Descriptors help files

[MattS6464]

Updated the help files for the objects in the descriptors folder. Also created xml files for each object.

[AlexHarker]

entrymatcher(~) now has an embed attribute to save the data with a patch.

There might be some other new messages also (search for class_add() in the source to see messages - note that messages marked A_CANT don't need documentation - they are invisible max things).

[MattS6464]

The updated entrymatcher(~).xml files are still incomplete.
Every message/attribute has now been added to the xmls but some of them have a placeholder digest and description saying TEXT HERE where I wasn't entirely sure what to write.

The placeholder messages/attributes are also still missing from the help files and so will need explaining there too.

[MattS6464]

Corrected the methods in the xmls to correctly show the types for each method.
As above, some methods are still labelled with TEXT HERE.