window: Plug a small mem leak

==18302== 19 bytes in 2 blocks are definitely lost in loss record 2,961 of 9,265
==18302==    at 0x4027AB1: malloc (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==18302==    by 0x51558F9: g_malloc (gmem.c:97)
==18302==    by 0x5155C10: g_malloc_n (gmem.c:332)
==18302==    by 0x516FB80: g_strndup (gstrfuncs.c:418)
==18302==    by 0x5149592: g_key_file_parse_value_as_string (gkeyfile.c:4112)
==18302==    by 0x5145DEE: g_key_file_get_string_list (gkeyfile.c:1935)
==18302==    by 0x8069243: ar_conf_get_string_list (ar-conf.c:774)
==18302==    by 0x8062004: install_recently_played_menu (window.c:1181)
==18302==    by 0x8062A9B: game_type_changed_cb (window.c:1514)
all: Add QtSvg based renderer for KDE card themes

The old one using librsvg is quite slow and doesn't work for most
of the KDE card themes. Add a new renderer for KDE card themes using
QtSvg.

Also add an alternative QtSvg based renderer for our native
card themes. However, most of our card themes don't render correctly
with it. Apparently QtSvg requires that for xlink:href="#foo" refs, it
has seen the id="foo" on an element *previously*, instead of it existing
anywhere in the document.