Make "backup is online" notification an error one
Do not use a constant name for the error image. This will be set by the theme
Allow to set a dependence on a StateType object

A dependence is based on the StateType being visible, and all the children of that state to be visible
Make consistent use of InfoMap type
Create MythUIComposite class.

MythUIScreenType, MythUIGroup and MythUIStateType all inherit from it. It allows to use SetTextFromMap and templates on all those composite elements and their children objects.

Fixes #11680
Define StateType when a MythNotificationError is displayed

This allows to define the behaviour on how to display errors in the theme
Make default theme use new error state type.
Add error.png logo, designed by Artist: Kyo-Tux
http://www.iconarchive.com/show/phuzion-icons-by-kyo-tux/Sign-Error-icon.html
Update MythMusic use playback registration API

Following 5201ed1cef70fe3e6e621733c585617bb8749dd0, MythMusic lost the ability to stop / resume music playback when TV/video playback starts. This restore this functionality. Also now compatible with AirPlay