Move OC_Defaults to OCP\Defaults

* currently there are two ways to access default values:
  OCP\Defaults or OC_Defaults (which is extended by
  OCA\Theming\ThemingDefaults)
* our code used a mixture of both of them, which made
  it hard to work on theme values
* this extended the public interface with the missing
  methods and uses them everywhere to only rely on the
  public interface

Signed-off-by: Morris Jobke <hey@morrisjobke.de>