bitbake: hob: create save image dialog used to save a template

bitbake: hob: create save image dialog used to save a template

Implemented a new dialog used by Hob. This dialog was desinged
in order to permit to save only in a particular directory.
Also, it has a field where the user can type a description
for the image.
Implemented in the handler a method to retrieve the topdir variable,
because the changes will be saved in {topdir}/recipes/images directory.

[YOCTO #4193]
(Bitbake rev: 117d4809a62e28ffe7e9dcda5433993d76f7d934)

Signed-off-by: Cristiana Voicu <cristiana.voicu@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>