add build artifacts table and other improvements

add build artifacts table and other improvements

We add a BuildArtifacts class to store data about files
discovered during the build process and not stored anywhere
else.

Small cosmetic changes in the toasterui.

Add model methods to return file path display data relative
to the build environment instead of absolute file paths.

[YOCTO #6834]

Signed-off-by: Alexandru DAMIAN <alexandru.damian@intel.com>