GList Hunt

Wiped out all GList instances, except in libcroco[1] and libnrtype[2].

Wiped out all GSList instances[3]

Fixes like a hundred memory leaks.

[1] and 2 occurrences in sp-style-elem.cpp tied to libcroco

[2] and 1 occurrence in widgets/font-selector.cpp tied to libnrtype

[3] except where it introduced a crash: tentative diff was https://gitlab.com/marcjeanmougin/inkscape/snippets/1677547 but crashes. This is probably because our code for actions is too unconventional and bad enough in gtk, and gtkmm fails some casts :/