Implement interface to select game installation location

[ShadowApex]

trafficstars

Currently OpenGamepadUI doesn't provide an API that can allow Library plugins to prompt the user where they would like games to be installed (e.g. installed on SD card vs internal disk). We should provide an API interface to allow users to select where to install games. Standardizing this interface could be difficult as different library providers may have different ways of storing games. At very least, we should give Library plugins an optional content box where they can populate an interface to select storage.

[pastaq]

We should have a DiskManager utility that uses the steam-removable-media format to manage disks within the UI.

Features I'd like to see:

• Default install location (per library)
• Default install location (plugins)
• Format, and eject media
• Inspect drive for usage, mount point, and SMART data.

[pastaq]

this will be partially implemented with #368