About dialog: Keep the build info in the clipboard even when MPC-HC is closed.
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.