Add an API for accessing the photo gallery

[freakboy3742]

What is the problem or limitation you are having?

Most platforms (especially mobile platforms) have a photo gallery/photo sharing capability. Toga should offer a cross-platform API to:

1. Select a photo from the device's photo gallery; and
2. Save an image to the device's photo gallery.

Describe the solution you'd like

A cross platform API (toga.App.gallery?) that allows users to save/retrieve photos from the device's photo gallery.

Describe alternatives you've considered

Additional context

On mobile devices, this will likely involve additional permissions.

On desktop platforms, it may be no more complex than opening a file dialog on the ~/Photos folder (or similar location). It might be desirable to have a custom "photo browsing" dialog, rather than a "raw" file dialog.

[hkdkfih]

yes please