[FEATURE] Add sortable attribute to models

Usage:

* sloth\sortable

Adds manual sorting on the BE for the model

* sloth\sortableOnRelation

Adds manual sorting but only for relations (inline). This will speed up
orderings on BE if model is only being used from inoine relations as
stated in [1].

[1]
http://docs.typo3.org/typo3cms/TCAReference/Reference/Columns/Inline/Index.html#columns-inline-properties-foreign-sortby