Display thumbnails for image URLs found in developer comments.

Thumbnails are clickable and will open in a lightbox window. When
several images are found for a developer comment, the images will
be available as a gallery.

Fixes bug 2528.
Use 'focused' class in the search form.

This will allow for emulating a :focus selector in the whole form.
Display subtle shadow when focusing the search input.
Hide search options popup when pressing the ESC key.

Fixes bug 2744.
Added more space between the search input and the popup.