[FEATURE] Introduce handling of missing files

To keep track of missing files we introduce a missing
flag is sys_files.
When indexer detects file is missing the flag is set
and reset when file is present again.

In backend a message is shown when a reference to a
missing file is used.

Resolves: #50827
Change-Id: Iaaf9455698507c5cd574f6928fa5a563a49ad121
Reviewed-on: https://review.typo3.org/22913
Reviewed-by: Steffen Ritter
Reviewed-by: Kai Ole Hartwig
Tested-by: Kai Ole Hartwig
Reviewed-by: Mattias Nilsson
Tested-by: Mattias Nilsson
Tested-by: Steffen Ritter