Gallery view: open image, go back to home

[kirillt]

At the moment, we have only "Grid" view displaying all images at once.
"Gallery" view should display the big picture of a single image.

User should be able to:

1. Enter "Gallery" view by double-clicking an image from "Grid".
2. Leave the "Gallery" view and go back to the "Grid".
3. Change the image to display by going forward or backward.