Add PNG support

For this I had to change the backup method. Now all backups are restored
during initialization, and each game stores its own backup just befor
the overlay is applied.
Split Users file
Split games file
Split download file
Split overlays file
Split backup file
Move function GetSteamInstallation to users.go
Add support for non-Steam games

Missing features: using the user-chosen label and finding their tags
Full non-Steam games support