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This is so scrollbars can be themed throught the "trough" and
"slider" classes.

This is also implemented in the theming engine, although the default
CSS should be using it in order to theme buttons, troughs, sliders
and such.

Several classes and regions may be set to GtkStyleContext, so rename
the functions to be more intuitive.

Comparing a selector with a widget path was being done backwards since
the change to use GtkContainer::get_path_for_child().

It actually messed up with the state being actually set, and having a
transition animation running for it. Now this dichotomy is removed, and
gtk_style_context_state_is_running() only checks whether an animation is
running, leaving state checking to flags & GTK_STATE_FLAG_*.

Both API and file format is documented, there's still missing
a migration doc though.

The case for paned can be guessed out from the height and width,
for handlebox the orientation parameter doesn't make much sense,
and this way it could also be used for resize grips.

The css parser has been modified to parse correctly radial gradients:

background-image: -gtk-gradient (radial,
                                 center center, 0,
                                 center center, 0.8,
                                 from (#000), to (#fff));

The theming engine has been modified to correctly animate these,
as well as transitions between different pattern types.

This is now used throughout in order to have the full path for a given widget,
including intermediate named regions, the default implementation just returns
the GtkContainer's path copy, no intermediate regions between.

Invalidation region coalescing now happens recursively as draw() runs, and the
widget's x/y as now drawing coordinates are 0,0 based.

Now other CSS files can be referenced from the currently parsed file:

@import url (other-file.css);
@import url (/some/file.css);

This is done anyway on each iteration to parse_rule() in parse_stylesheet()

It will now return a full path, and check about the file being sane.

Given there are other rules such as @import (which will be supported),
keep a sane namespace here, so for (re)defining a color name, in the CSS
file it will look like:

@define-color color-name #fff;
@define-color other-color mix (@color-name, #f00, 0.4);