Missing floating point number type support in config schema

Problem/motivation

Config schema supports integers but not floating point numbers. See https://drupal.org/node/1905070 for the docs and fully type list. The responsive preview module (#1741498: Add a responsive preview bar to Drupal core) would need a floating point number expressed for dots per pixel which for a Google Nexus is 1.325 for example.

Proposed solution

Wire in support for floating point numbers. Should hopefully be easy with typed data already supporting this(?)