Move this to a separate header, since it is not parser functionality, and kill the duplicate gtkcssparserprivate.h header.
This commit is essentially a large reorganization. Instead of all value subtypes having their own compute function, there is the general _gtk_css_value_compute() function that then calls a vfunc on the subtype.
It's completely replaced by GtkCssNumberValue now.
.. and parse border-image-slice with it.
