Refactor ET_Set_Field_File_Tag_Picture()

Rename to et_file_tag_set_picture(). Rather than handling a EtPicture
**, take a File_Tag and set the picture field appropriately.
Use new et_file_tag_set_picture()
Refactor et_picture_new() to be more complete

Make et_picture_new() both create a new EtPicture instance and
initialize all the fields. Copy the description string and ref the image
data GBytes. Adapt existing users (and tests) to the new interface.