Change old strings to MPC-HC.

Note that the file association related strings are left intact for the time being.
Also the mutex is the same.
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.
installer: set the ExePath after resetting to prevent re-migration.