Make session loader store unused property keys

[milasudril]

This is important if new properties are needed for future versions and old versions should not destroy any data written by newer versions.

This can also be used to store more metadata.

[milasudril]

After some reasoning, this cannot be done without breaking compatibility with the current session file format. The main issue here is that the session file format uses literal bit fields, and an old decoder cannot have any idea of what bit an unknown identifier represents. Thus, this issue will be pushed to the future. For the more visible issues to not be blocked on this issue, forward compatibility is not supported throughout the 2.0 series.