Fix constant attr inheritance & defined_value conditional default

[Pablete1234]

Fixes the fallback attribute not being respected if defined like:

<constants fallback="true">
  <constant id="a">b</constant>
</constants>

Additionally the default constant-comparison will be defined value instead of defined, as it seems like the more reasonable default as you probably don't want to include a block if it's deleted. This could be considered a breaking change but it's been in dev for literally a day, so i think it will be fine