gtk-gui.c: Move the download dialog related code to a new file

A new file download-dialog.c now contains all code related
to the download dialog, which was previously defined in gtk-gui.c.

Also, a new file callbacks-gtk.h now has two macros
OPTIONCALLBACK, UNITCALLBACK shared only between
download-dialog.c and gtk-gui.c.

Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>