Child style classes are like style classes, but with additional positional
information. This would be usually set by the parent container.

Style classes are the replacement of detail strings.

GtkThemingEngine will be the theming engines base class, with default
implementations for all paint functions, and readonly access to the
related GtkStyleContext data.

This relates a GtkStyleContext with a widget path, so
all style querying would happen based on it.

This function composes and returns a GtkWidgetPath representing
the passed widget.

GtkWidgetPath represents a widget path. It will mainly used by theming
engines to know detail about the widget hierarchy without actually
accessing the widget.

There will be one GtkStyleContext per widget, at the moment its
lifetime is tied to the widget's, but it could be narrowed down
to GTK_WIDGET_REALIZED.

It basically represents GtkStateType as a flag set.

Also some properties have been added as a default set.

GtkStyleContext will conglomerate the information of several
GtkStyleProviders for widgets and theme engines to query it.

As simple as it can get.

The get_style() function isn't final yet, further parameters
should be added to query details based on the widget hierarchy,
name, etc.

At the moment there's only basic API for adding,
removing and clearing a property. There's also
API to merge 2 GtkStyleSets.

Be consistent in the naming of objects.

We argued replacing GdkWindowClass with a input_only boolean, but this
is not doable in an API-stable way, so I decided against it.

And other minor gdk doc fixes.

Pointed out by William Jon McCann.