Draft: Port window state saving to KConfig.WindowStateSaver

Part of frameworks/kconfig#12

Why draft? Needs to wait for Frameworks 6.7.

Reason for the change

Now that we have it, let's use it! Less custom code for us to maintain here, and the data gets saved in the state config rather than the settings config.

Test plan

1. Launch app -> it opens at a sane default window size
2. Launch app, resize window, close app, re-launch app -> size remembered
3. Launch app, resize window, maximize window close app, re-launch app, de-maximize window -> maximization state and old window size remembered

Screenshots or screen recordings

N/A

Bugs fixed

N/A