Add generalized (non-Field) Views field handler other modules can use

Currently a Views field handler is provided for the Fraction Field (based off of the views_handler_field_field class).

This works great for the Fraction Field, but I would like to make it possible for other modules to define their own two-column fraction database storage schemas, and provide a general-purpose Views field handler for them to use.

This is going to be used here: #2607402: Views integration for sensor data