Updating Cheats manager when the ISO is changed etc.

### Non-modal dialogs

The Cheats manager dialog was made non-modal in 983d5d1f

The Cheats manager should be updated when the ISO is changed because it's non-modal

Disabling Cheats manager menu item when an emulation isn't running because it doesn't have a use then

Removing code that read the ID from file because the cheats manager only has a use when an emulation is running (when the ID doesn't need to be read from the ISO again because it's in a variable). This fixes loading Gecko codes for .wad during an emulation because "VolumeHandler::GetVolume()" return false in this case