Alphabetical properties within classes

[nickevansuk]

As this PR shows https://github.com/openactive/models-php/pull/89/files occassionally schema.org change the order of the source properties when they change their approach to generating their schema files.

When this happens the PR becomes bloated with reordering changes, which makes it much harder to find issues.

A simple solution to this is to sort the properties that are outputted by the generator alphabetically (as there is no inherent ordering to them in schema.org). This will ensure that they are not affected by such changes, and make it much easier to debug any resulting issues.