Feature: Switch to single instance with multi-windowing support

### What feature or improvement do you think would benefit Files?

Currently, each instance of Files only supports a single main window (in `App.Window`), and tab tear-off is implemented by starting a new instance through URI activation. Allowing multiple windows will make this faster and more lightweight and opens the door to having only one instance of Files running at a time (like explorer).

### Requirements

- This proposal will allow a single app instance to have multiple main windows.

### Files Version

2.5.1.0

### Windows Version

10.0.25387.1200

### Comments

This will be a very large effort