[StThemeNode] Add support for 'width' and 'height' CSS properties.

[StThemeNode] Add support for 'width' and 'height' CSS properties.

The behavior in respect to borders matches CSS - the properties set the size of
the content exclusive of the borders (CSS3 box-sizing property - not implemented
here - changes this).

min-width/min-height correspond very closely to the CSS meanings.

width/height are a little different from the CSS meanings - the CSS meaning is
"exactly this size unless overridden by min/max-width/height" - but within the
realm of our layout algorithm, making them control natural size is pretty
close.

This way we can force elements to have a fixed natural or minimum size.

https://bugzilla.gnome.org/show_bug.cgi?id=598651