Been doing some more gc-related work; I've been taking advantage of the facilities offered by the garbage collector to simplify SPRepr's implementation a little.

The biggest win was in the strings used in the repr transaction logs. Previously, new copies were made to put in the log since memory management got hairy otherwise. Now the transaction logs and the attributes/text nodes can share the same strings.

Another win is that SPRepr does not require a destructor anymore.

Some current rough areas:

* because finalizers are needlessly registered for SPRepr (as an Inkscape::Refcounted), we will occasionally end up with finalization cycles. This is not really harmful (especially for an object which has no destructor), but it does print a message on stderr and leak a very tiny amount of memory when it happens.

I need to figure out how to have finalizer registration for Inkscape::Refcounted be optional.

* secondly, the shared, immutable, garbage-collected strings allocated with Inkscape::Util::shared_c_string() are currently indistinguishable from any other C string.

I am wondering whether I should introduce a special class for these, so type-checking will prevent someone from passing an ordinary string when they should be passing one of these instead (passing a static string is OK, but a temporarily allocated string will cause interesting and hard-to-diagnose bugs when it is freed out from under someone who was still holding a reference because they assumed it was safe to do so...)

-mental