gst/matroska/: Reverse the level list as we usually are only interested in the first element or want to add a new fir...

Original commit message from CVS:
* gst/matroska/ebml-read.c: (gst_ebml_level_free),
(gst_ebml_finalize), (gst_ebml_read_change_state),
(gst_ebml_read_element_level_up), (gst_ebml_read_master):
* gst/matroska/matroska-demux.c:
(gst_matroska_demux_parse_contents_seekentry):
Reverse the level list as we usually are only interested in the
first element or want to add a new first element. Having the
first element stored at the end and calling g_list_last() and
g_list_append() is more expensive.
Also use GSlice for allocating the GstEbmlLevel structs.