toc: expand GstTocEntry with loop fields

Add loop_type and repeat_count fields to GstTocEntry plus setters and getters.
This allows to represent edit-lists in a toc as well as loops in instruemnts (wav, xi).

API: gst_toc_entry_set_loop
API: gst_toc_entry_get_loop
basesrc: DO_NOT_PUSH - support for own pools