frontend, obs-filters: Add missing file support for filters

[exeldro]

Description

Add missing file support for filters

Motivation and Context

When importing a scene collection you get a popup with missing files which does not contain all missing files.

How Has This Been Tested?

On windows 11 with a mask filter and remove the mask from disk

Types of changes

• New feature (non-breaking change which adds functionality)

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.

[PatTheMav]

Seems fine apart from code style changes.

It feels a bit "wrong" having to implement this frontend concern in the filters themselves (instead the frontend should be able to look at the "resources" used by a source and thus check for its existence), but this is how the system currently works in OBS for better or worse.

[exeldro]

Rebased and applied suggestions from @PatTheMav