Use GBytes for image data inside Picture

Avoid many superfluous memory allocations when copying Picture instances
by using GBytes instead of a bare pointer, and incrementing the
reference count when copying. Use the slice allocator to allocate
Picture instances. Avoid Picture->bytes from ever being NULL. Update the
Picture tests.

Inspired by a report of excessive memory usage on the mailing list:

https://mail.gnome.org/archives/easytag-list/2014-December/msg00000.html
Make EtPicture a boxed type

Rename Picture to EtPicture and define a boxed type with
G_DEFINE_BOXED_TYPE. Rename Picture functions to et_picture_*(). Fix a
memory leak when destroying the tag area tree view, by using an
EtPicture column in the list store. Update the EtPicture tests.
Fix memory leak when saving a temporary FLAC file