Fix wrong path in history when opening files from a network drive.

This old bug has been made visible because of 3fc006ff775f7bf8c29da82c33069d47f6ea47ea.

This commit fixes #3594.
Factorize the code from file chapters handling.
Factorize the code from DVD chapters handling.
Skip to previous chapter: Use the same threshold for files and DVDs.

Previously thresholds of 3 seconds and 7 seconds were used respectively for files and DVDs, to choose between restarting a chapter or skipping to the previous one. Now both uses a 5 second threshold.
Options dialog: Disable "Reset" when the INI is read-only.

Ref #3591.