Parse render nodes from text files

Parse render nodes from text files

Instead of the previous approach using GVariant, this new approach uses
human-readable text files as the serialization format for render nodes.

The format is a custom one, but it is inspired by QML and conforms to
the CSS syntax. Because of that, we can use the CSS machinery from GTK
to parse it, and in particular share code to parse properties that GTK's
CSS machinery also supports, such as colors.

This commit breaks all existing usages of node files - such as the
testsuite and various test tools - they will be fixed in further
commits.