Allow laying out content beyond standard areas of the window

### What is the problem or limitation you are having?

On iOS, e.g. the Photos app displays images fullscreen, having content behind the status bar.

There might also occasionally be use-cases where, say we'd like a slightly tinted colored background extending all the way up.

### Describe the solution you'd like

A platform-specific API for extending content and layout beyond the area into the navigation bar and status bar.

An example design could include allowing the users to modify the top_offset of the container, and providing conveniences like exposing the height of the status and navigation bars, helping users decide exactly how much to extend. We could also expose values of these kinds for users to adjust layouts of certain things appropriately. [edit]

### Describe alternatives you've considered

Do nothing

### Additional context

_No response_

Start by surveying Toga's existing window and container layout APIs, then review the iOS-specific handling of the status and navigation bars. Done means defining a platform-specific way to extend content into those areas and expose the relevant bar dimensions or offsets for layout adjustments.

Written by the indexing model from the issue text.