toastergui: improvements in layer selection logic

toastergui: improvements in layer selection logic

This patch clearers and bring fixes for the layer selection
logic in order to enable information collected during build to be used
in configuring projects, specifically targeting the recipes
learned through the building process.

The patch also adds tests to verify the layer selection logic.

[YOCTO #7189]

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