Inform the user when saving an image

~~The current saving function writes to the console to say the image was saved. It should show a dialog with text saying whether it saved successfully(and the path, if so) or if there was an error, say so and log the exception.~~

Instead of a dialog, it would make more sense to do it like vim and show the message in #15's input after writing `:w`, so this issue now depends on #15 being completed.