mp_tags: move generic mp_tags stuff into its own .c/.h files in common/

rename add_metadata to the more genera/descriptive mp_tags_copy_items_from_av_dictionary
talloc README: more human readible api reference
vf: add vf_control_by_label to send vfctrl to specific filter
vf: auto-label video filters if no label is specified.
vf_lavfi: fix ffmpeg deprecation warning for avfilter_graph_parse

use avfilter_graph_parse_ptr for ffmpeg
vf-metadata: copy AVFrame metadata into vf_lavfi priv

store it as mp_tas and add VFCTRL_GET_METADATA to access it from elsewhere
vf-metadata: add vf-metadata property

This is a read-only property that uses VFCTRL_GET_METADATA
to retrieve mp_tags metadata from a filter specified by label
vf-metadata: lua cropdetect example script
vf-metadata: test script
af_lavfi: fix graph parse deprecation warning
af_volume: fix clang -Wsometimes-uninitialized