Support filtering nested structures by tag

Allow tagging a nested property structure with
`blueprint:"filter(key:\"value\")"`, which will only allow property
assignments to properites in the nested structure that are tagged
with `key:"value"`.  Moving the filter into Blueprint instead of
the project build logic allows more reliable and consistent error
messages.

Change-Id: I06bc673dde647776fc5552673bdc0cdcd7216462