lib/bb/ui/crumbs: Add HobAltButton and use it

lib/bb/ui/crumbs: Add HobAltButton and use it

This adds a gtk.Button subclass for secondary actions which unsets the
relief so that the user is aware that the button is clickable and yet it is
more subtle than the primary action (such that the primary action is the
most obvious one) - this is per the interaction design.

Further we replace all uses of gtk.LinkButton with the new HobAltButton

Partially addresses [YOCTO #2105], a follow on patch will theme the button
so that it matches the visual design.

(From Poky rev: c3f17fed243180678264168968333463b203bfa0)

(Bitbake rev: 601521c2d7f5568d94529a77b2cbe19fef7cbf48)

Signed-off-by: Joshua Lock <josh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>