Port our imported parts of Mx to ShellTheme

ShellTheme replaces both StStyle and ccss_stylesheet_t.

The interface StStylable is replaced by usage of ShellThemeNode.
A concrete node class allows some significant optimizations of property
inheritance that would have been much more difficult to achieve with
the highly abstract pair of StStylable and ccss_node_t.

Some operations that were previously on StStylable (like the
::style-changed signal) are directly on NtkWidget.

Custom properties are no longer registered as param-specs; instead you
call directly into shell theme node to look up a length or color:

shell_theme_node_get_length (theme_node, "border-spacing", FALSE, &spacing);

The dependency on libccss is dropped, while preserving all existing
functionality and adding proper parsing and inheritance of font properties
and proper inheritance for the 'color' property.

Some more javascript tests for CSS functionality are added; workarounds for
a CSS bug where *.some-class was needed instead of .some-class are removed.

https://bugzilla.gnome.org/show_bug.cgi?id=595990