QSettings stored at multiple locations

[slipher]

We use the QSettings to store some settings for the updater. But there is also some Qt support code that makes its own QSettings in a different place to store settings for "QQControlsFileDialog". We should try to make all of them be in one place.

• Windows: Our settings are in the registry at HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Unvanquished Development\Unvanquished Updater while QQControlsFileDialog is at HKEY_CURRENT_USER\SOFTWARE\Unvanquished Development\Unvanquished Updater.
• Linux: Our settings are in the file ~/.config/unvanquished/updater.conf, while QQControlsFileDialog is in ~/.config/Unvanquished Development/Unvanquished Updater.conf.

A potential complication is that our main settings location on Windows requires admin permissions to write. But this would only become a problem if some component used in the splash screen were to need QSettings.