styleproperty: turn border-image into a shorthand property
It's composed by
- border-image-source: a cairo_pattern_t holding an image from file or a
gradient
- border-image-slice: a GtkBorder containing the slice offsets to apply
on the image
- border-image-repeat: a GtkRepeatType for the image
We deviate from pure CSS3 in the following ways:
* border-image-width is assumed to be always 1, i.e. always equal to
what's specified by border-width. I don't think it's a particularly
useful property to have, but we could add it later if needed.
* border-image-outset is absent, as we can't render outside of the
allocation yet.