[Settings] Settings file locations determined by QStandardPaths.

This change makes it so the settings files for Windows users will be in the standard location for Windows. It uses QStandardPaths to do this. I did not code any settings migration from the old location, so for Windows users it will result in a loss of settings, though a small copy operation could be added to the installer to take care of that. For Linux users, this should have no effect, the settings should still end up in the same directory, however I have not tested it.

Also note that due to the enum values used, this now makes Qt >= 5.5 required.