Add ability to create context menus for Widget

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

Currently, Toga seems to provide no way for us to create context menus and associate them with certain widgets. This limits the potential to develop some more complex applications that might benefit from them — apps containing multiple panels that can be placed in different sides of the GUI, as an example, like image editors.

### Describe the solution you'd like

Add ability to create context menus and assign them to certain widgets, like Boxes. It's especially useful because, since Toga doesn't provide drag and drop support yet, context menus can provide a way to add options such as "Move panel to the right/left" etc.

### Describe alternatives you've considered

None

### Additional context

_No response_

The issue names no files, tests, or entry points. Start by locating Toga's widget and menu-related entry points, then determine how context menus should be created and assigned to widgets such as Boxes. Done means widgets can expose the requested context-menu behavior, with coverage for the supported usage.

Written by the indexing model from the issue text.