Putting the crash sentinel in the main "obs-studio" directory carries
the risk of unwanted deletion of files in the same directory, which
usually contains user configuration files like "user.ini" and
"global.ini".
Even though the code will ignore any file that does not start with the
crash sentinel prefix string, the code itself would allow changing the
prefix to "global.ini" and thus inadvertently delete the global
configuration file as well.
To further reduce the risk, this change will put the sentinels in a
separate sub-directory that should only ever contain sentinel files,
all of which can possibly deleted without any data loss for the user.
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