Support standard CSS's `voice-family` property

[bertfrees]

Pipeline supports voice-family, but a custom version of it. Make it compatible with standard CSS.

See https://www.w3.org/TR/css-speech-1/#voice-props-voice-family

[bertfrees]

As part of this issue, we will also do some refactoring and eliminate some XSLT code. The voice-family property will be parsed in Java to a VoiceFamilyList object, and this object will be passed to VoiceManager.findAvailableVoices(). The SSML will only have to contain "voice-family" attributes, instead of a combination of "voice-gender", "voice-age", "voice-selector1" and "voice-selector2".

Possible values of the voice-family property are:

• A prioritized list of component values that are separated by commas, with each value either identifying a voice or engine name, or specifying the gender (male, female or neutral), optionally the age (child, young or old), and optionally the preferred variant (1, 2, etc.).
• The value initial: the CSS spec says the default value is implementation dependent, and we choose to make "initial" equivalent to an empty list of component values, which means that the VoiceManager makes its selection based only on the locale
• The value preserve: we will not support this value for now
• The value inherit