DRM panel orientation property

Add support for the KMS panel orientation property. This property allows a KMS driver to statically declare a fixed orientation for a connector. This is useful in cases such as mobile phones or embedded displays which can have displays mounted at an unexpected orientation.

When the property is parsed, the backend just sets the member on weston_head, and no further action is taken.

The default Weston frontend has been modified to take it into account during output configuration: if an output only has a single head, it will default to using the transform declared by the head, unless overridden by the config file.

This series was originally a single patch from @lynxeye. I have taken the core parts (support for a transform member in weston_head, support for setting it from the panel orientation KMS property) and rewritten the configuration. @marius.vlad0 submitted it as !301 (closed) but I wanted to rewrite the config aspect; the original series always override the output transform inside libweston, maybe later resetting it in Weston's configuration.