lib/bb/ui/crumbs: replace HobXpmLabelButtonBox with HobImageButton

lib/bb/ui/crumbs: replace HobXpmLabelButtonBox with HobImageButton

HobImageButton is an gtk.Button subclass, and therefore behaves like a
button with prelight and focus states, with an icon and two lines of text -
primary and secondary. The secondary text is displayed in a lighter colour
using a new module method, soften_color(), per the design.

(Bitbake rev: b91cc96c4ff4195ac26fdfd1fb0c2ff8db06aff8)

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