Change GString into Glib::ustring

Merged username-removed-1587188 requested to merge jali/inkscape:ustring_refactor into master

Removes every use of GString. (Except for the files in libcroco/ , which are untouched for obvious reasons.) This fixes multiple cases of incorrectly (or not at all) freed GString*. Note that the code still contains usage of char* and gchar*.

Edited by username-removed-1587188