Fix saving of target platform

Don't only save the target platform on allocation: we want to save on
save before allocation too.
Fix nodes not displaying any target platform

When prettrifying the target platform, we were not using our internal
method that falls back to the default platform, resulting in the pages
showing nothing at all as target platform.
Merge pull request #1098 from vuntz/target-platform

Fixes related to saving and displaying the target platform