Extends Reader to support switching between Jumps using keyboard

Adds a new action for handling pressing the Tab key, we handle it inside our ImageBrowser by switching between jump objects on the current page. We use an index called currentJumpIndex which indicates the current selected jump, to switch between the jumps.

I'm not sure why I can't access the image's function nextJump() from within the ListView itself, so writing something like this would result in an error stating that image is not defined:

function onTabPressed() {
    root.currentItem.image.nextJump();
}

So, I had to make a function inside the Flickable by which I call the image's function. We'd need to add a new handler for the clicking part, but I feel like calling the functions like this is a little awkward.

@leinir