hob/settings: implement a new tab in settings dialog to show SSTATE_MIRRORS

hob/settings: implement a new tab in settings dialog to show SSTATE_MIRRORS

Add a new tab to show correctly SSTATE_MIRRORS variable. Now you can add new
mirrors or delete mirror. "info" image was also changed( it is smaller, so it
can be next to labels).
>From "Build environment" tab, SSTATE_DIR and SSTATE_MIRRORS vars were removed.

[YOCTO #2893]

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