Move WindowManager outside of WidgetsApp (#188866)

This is a prototype branch for moving `WindowManager` outside of
`WidgetsApp`.

Our current approach where first window is "blessed" and other windows
are anchored inside first window hierarchy is causing multiple issues:
- Only the first window has navigator
- Secondary windows focus zones are inside primary window focus zone so
it is always focused.
- Widget inspector only works in primary window

In general there is no reason for one window to be special. Normally for
a desktop application all windows should be equal. The closest we can
get to this is to give each top level window its own
Widgets/MaterialApp. While this duplicates some elements that should
normally be shared, arguably it behaves better than doing what we do
right now.

Ideally we'd want to split the App widget into shared and per window
parts, but that will be a much bigger change.

With this branch the top level Dialog/Regular window have each their own
MaterialApp widget and separate focus trees, solving the issues above.

`Popup` and `Tooltip` window work and are anchored at proper place in
window hierarchy (they should not be top level window). However we might
want to consider a convenience Widget to anchoring these instead of
using `ViewAnchor` + `View`.

Screenshot of example app running in this mode - notice the Debug banner
and widget inspector buttons in each window.

<img width="900" height="800" alt="Screenshot 2026-07-01 at 5 48 13 PM"
src="https://github.com/user-attachments/assets/bbb6e646-e9d8-4618-9702-19afe0749b11"
/>