Change old strings to MPC-HC.
Change "Gabest" to "MPC-HC".
Installer: change more strings and remove the old keys when uninstalling.
CMPlayerCApp: Move registry settings from the old to the new location.

Only when we're initializing the MPC-HC key for the first time.
CAppSettings: Add versioning to settings and update any old versions.
FileAssoc: Revert "Change old strings to MPC-HC."

Partially reverts 4021dc9150b339f6d80c382b2d79d2e5b9434944.
installer: set the ExePath after resetting to prevent re-migration.