frontend: Only saved scene sources from frontend-managed canvases

[dsaedtler]

Description

Changes the saving logic a bit so that scenes from canvases not managed by the frontend won't get saved.

Also refactors the saving a bit since the separate GenerateSaveData() seems unnecessary.

Motivation and Context

The intention was that the frontend should only save and load canvases it manages (i.e. created via the frontend API). Likewise it shouldn't save any scenes on canvases that aren't frontend-managed or ephemeral.

This problem was reported on Discord.

How Has This Been Tested?

Saved and loaded some scene collections.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• Tweak (non-breaking change to improve existing functionality)
• Code cleanup (non-breaking change which makes code smaller or more readable)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.