Callback functions for window resizing

[eschricker]

Checklist

• I have searched the issue tracker for open issues that relate to the same feature, before opening a new one.
• This issue only relates to a single feature. I will open new issues for any other features.

Is your feature request related to a problem?

I want to persist resizing changes made by users. If the user opens the application and resizes the window this settings should be saved. The next time the app will be opened, the window should have the same dimensions.

There are related issues like #2563 or #554 but they want the persisting capabilities within the framework.

Is it possible to construct a solution with the existing API?

It's currently not possible. You can't register a callback which is called if the window is resized.

Describe the solution you'd like to see.

Add a new function callback OnResized(size fyne.Size) to the interface Window which is triggered if Resize() is called.

With this I am able to catch window resizing and save them via the preferences api.

[eschricker]

I have found the solution mentioned in #224

This works fine for my use case!