hig.py: refactor into individual components

Since hig.py was becoming too large (which impacted maintenance and
operations like git merges) it was split into individual files for
the classes that were implemented in hig.py. hig is now a Python
package (lib/bb/ui/crumbs/hig/). The patch was tested by
building core-image-basic/qemux86 in Hob and accessing the various
UI elements implemented in hig. Note that the patch does not change
the functionality of Hob in any way, it's just a code refactoring
that should make life a bit easier for Hob developers.

[YOCTO #3200]

Signed-off-by: Bogdan Marinescu <bogdan.a.marinescu@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>