Storage manager

[JosefNemec]

Basically the same thing as Steam's new storage manager. #167 should be done together with this.

[erri120]

I've got some requests to update my Game Mangement extension which implements this in a very hacky way:

• size calculations happen at application startup (non-blocking) and when a new game is installed/uninstalled
• size calculation is simple file enumeration and summation of file sizes and are processed in parallel using the Microsoft High Performance Toolkit
• storage info is saved as json and displayed in a very ugly and underdeveloped DataGrid

The extension also implements Uninstallation via simple Directory removal which goes hand in hand with size calculations.

Since these features are better off being part of Playnite itself, I'll look into integrating them however I suck at UI at hate WPF bindings so I won't be doing that.