cssnumbervalue: Remove gtk_css_number_value_get_unit()

cssnumbervalue: Remove gtk_css_number_value_get_unit()

This is in preparation for calc(), as calc(50% - 5px) is valid CSS
but has 2 units. Instead, add a function to query a value's dimension
(so we can differentiate lengths from numbers) and add a function to
query if the value contains percentages.