ShadowAssetManager dereferences attrs better

ShadowAssetManager now looks in the full style tree (children and
themes) while dereferencing parent styles.

In order, ShadowAssetManager will search:
 - All known children in this style path
 - The Theme Style
 - The ResourceLoader

Improves the logging information when a parent style can't be found.
Merge branch 'greg-baker-master'