Add ambience switcher action plugin

Here is a first version of the ambience switcher plugin, based on a copy of the profile switcher code, as shown here : http://talk.maemo.org/showpost.php?p=1454090&postcount=30

Limitations so far are that the ambiances path/name are hardcoded in the user interface, but they can be changed manually in the rule.qml file if needed.

Feels free to let me know of any change needed so that it fits with the rest of phonebot's code.

Thanks !