This is just lots of renaming.
The interface remains private, so the public API does not change, apart
from removing the definition of the Interface object to avoid
subclassing.
Previously, for compatibility with GTK 3.0, we allowed specifying
numbers without units and interpreted them as pixels, even when the CSS
specification didn't.
Remove that now that we can break API.
This requires adding code to do math on number values:
gtk_css_number_value_multiply()
and
gtk_css_number_value_try_add()
were added to achieve that.
Some tests are included.
