UI: Fix mem leak if --disable-missing-files-check

[kamalmostafa]

Description

Make the --disable-missing-files-check flag actually inhibit the collection of a files list, not just the display of the missing files dialog.

Fixes a memory leak of the files list itself when the flag is used (and there are some missing files).

Closes: #11074

Motivation and Context

Fix memory leak: #11074

How Has This Been Tested?

Manual testing of consistent behavior.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

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]

The bugfix is a bit messy, but that's because of the original implementation is messy to begin with:

• AddMissingFiles will automagically call obs_missing_files_create when obs_source_get_missing_files is called (because that function has two fallback paths that create a new struct)
• obs_load_sources is only used in this single instance in the entire application, so its only purpose is to provide the AddMissingFiles callback when loading sources
• This suggests that the check for missing files is an architectural fixture, not an "optional" feature (as the startup argument suggests)
• The memory allocated by obs_missing_files_create (which is always allocated somehow) is only explicitly released when ShowMissingFilesDialog is called:
  • Either no files are missing, so obs_missing_files_destroy is called directly
  • The OBSMissingFiles instance initialised is deallocated at some point (because it's destructor calls obs_missing_files_destroy.

This change only fixes the symptoms of the underlying architectural issue (e.g. the memory is allocated independently from UI functionality and thus should also be released independently from showing any dialogs), but it does avoid the memory leak.

[PatTheMav]

Please rebase this PR and give it another pass to make it ready for review/merging again. Cursory check makes this approach preferable to the one in #12628.