StThemeNode: simplify use of get_color/get_double/get_length

Although within St itself there are situations where the semantics of
these functions (return TRUE or FALSE and return the actual value in
an out parameter) is useful, it's mostly just annoying at the
application level, where you generally know that the CSS property is
going to specified, and there is no especially sane fallback if it's
not.

So rename the current methods to lookup_color, lookup_double, and
lookup_length, and add new get_color, get_double, and get_length
methods that don't take an "inherit" parameter, and return their
values directly. (Well, except for get_color, due to the lack of (out
caller-allocates) in gjs.)

And update the code to use either the old or new methods as appropriate.

https://bugzilla.gnome.org/show_bug.cgi?id=632590