Widget should have position and size properties

[mhsmith]

There are reasonable use cases for this information, e.g. https://stackoverflow.com/questions/78841211/how-to-get-the-dimensions-of-a-toga-canvas-in-python.

In fact, we're already using the non-public layout.content_... properties in two of the example apps (canvas and screenshot).

These properties should be the same as the existing properties in Window, except they should be read-only.