Add remaining declarations for working with GExiv2Metadata

[felixc]

See: https://git.gnome.org/browse/gexiv2/tree/gexiv2/gexiv2-metadata.h

Unimplemented:

gexiv2_metadata_open_stream
gexiv2_metadata_save_stream
gexiv2_metadata_save_external
gexiv2_metadata_set_xmp_tag_struct
gexiv2_metadata_update_gps_info

See also gexiv2_metadata_get_preview_properties in #3, gexiv2_metadata_get_preview_image in #4, and Exif thumbnails in #12.

I'm finding the missing declarations with:

diff <(sed -n 's/ *pub fn \(gexiv2_.*\)(.*/\1/p' src/lib.rs | sort) \
    <(nm -gD $(find /usr/lib/ -name 'libgexiv2.so') | awk '/ T gexiv2/ {print $3}' | sort) \            
    | sed -n '/\(_get_type\|gexiv2_metadata_from_stream\)/! s/> \(.*\)/\1/p'

[felixc]

gexiv2_metadata_get_tag_raw implemented in 5a0a314.

[felixc]

gexiv2_metadata_get_xmp_packet and gexiv2_metadata_generate_xmp_packet implemented in 56098a3.