Add a setting to hide the thcrap_loader window

With that setting, thcrap_loader would keep working normally, doing updates and all, but would not open its window (instead of opening it and then immediately closing it).

Because this setting removes the thcrap_loader window, it must be settable from the thcrap_configure window. In thcrap_loader, we may want to have either a popup (shown when the setting is enabled) or a clickable thing next to it, saying that it can be disabled in the thcrap configuration tool. Also, we probably don't want to allow having both this setting and the "keep updating in background" one, because that would result into having an invisible process lasting forever.

Requested by @zero318 about a year ago.

Start by tracing how thcrap_loader creates and closes its window, then inspect how thcrap_configure stores and edits the existing “keep updating in background” setting. Determine where the new visibility setting belongs and how the two settings should interact. Done means the loader continues updating without opening its window, the setting is configurable from thcrap_configure, and the conflicting background behavior is handled.