Implement auto-load/stream of most recent image

Fix #34 (closed)

The implementation is very simple and relies on #13 (closed). As a reminder, when a file is created in the current directory, a callback is executed in braggy to remove this directory from the cache and refetch the directory content.

I only added an optional load of the new image in this callback. In fact, most of the changes in this PR stems from the adding of the button and of a store to hold the (persistable) setting.