Move file picker into StereoKitC

[maluoi]

File picker is currently in Framework, and is not fully cross-platform compatible. For systems like UWP a custom file-picker UI cannot be used, since permissions are then not granted for that file. So, this needs:

• Native file picker for UWP
• Native 2D file pickers for Flatscreen
• Fallback StereoKit UI picker for MR platforms that don't have native pickers.

[maluoi]

Only remaining platform without a native file picker in flatscreen mode is Linux.

[maluoi]

Flatscreen file picker for Linux will come later. 🕙