Introduce @sortkey to support sorting annotation for a subobject

- `@sortkey` is a restricted keyword which allows to set an arbitrary sort value for a particular subobject
- `@sortby` is a restricted keyword which allows to set a reference to a property which in return will use the value of the reference property as sort value

`@sortby` and `@sortkey` are case sensitive, values are stored as a "string", and further will not appear visible to the end-user but subobject with a dedicated `_SKEY` will work the same way as a normal wiki page with a `_SKEY`.

## Subobject example
```
{{#subobject:
 |Has group=test
 |Has value=1111
 |@sortby=Has value --> _SKEY=1111
}}

{{#subobject:
 |Has group=test
 |Has value=9999
 |@sortkey=A --> _SKEY=A
}}
```

## Recurring events example
```
{{#set_recurring_event:property=Has date
 |start=January 4, 2010 05:00
 |end=June 8, 2011
 |unit=week
 |period=1
 |@sortby=Has date
}}
```

EDIT:PR only adds `@sortkey`