Allow property overrides and relocation in allOf/then blocks

[erral]

This PR fixes issue #25 where properties defined in the main 'properties' block were ignored if they also appeared in an 'allOf/if/then' block.

Key changes:

• Properties in 'then' blocks now merge with and override their base definition in 'properties'.
• Overridden properties are relocated to follow their gating question in the wizard flow.
• Support for multiple conditional definitions for the same key.
• Fixed a bug where shared keys in different 'allOf' branches were being skipped.

Verified with:

• Library unit tests (including new regression tests).
• Real-world templates from cookieplone-templates.

cc @pbauer for review.