[FEATURE] Support flux:form options.Fluidcontent.sorting

Setting options on FCEs for fluidcontent now sorts the elements in the new content wizard (per-group, thus not allowing you to mix groups):

```xml
<flux:form ... options="{Fluidcontent: {sorting: 1234}}"> ... </flux:form>
```

Note the use of an ExtensionName convention root-level option container - this is done to avoid collisions and confusion regarding consumers.