Quark map to hashtable

This is continuation of memory leak investigation in load_svg_cursor(). Changes involve elimination of std::map using GQuark keys. GQuarks are useful when list of string is static. With dynamic list (generated strings) using g_quark_from_string() is going to leak memory (see GLib glib/gquark.c:298). While removing quark keys I replaced std::map with a hash table, as it offers better performance (roughly 2x to 4x faster) for combinations I tested.

Also removed are GC anchor and finalizer from Inkscape Selection and related classes to aid ASAN in reporting leaks more accurately.