Multi image select

[nathanielrindlaub]

See: #25

[nathanielrindlaub]

Selection improvements:

• add a command + click listener for non-sequential image selection
• show context menu on right click if row isn't already selected? Finder does this and sets the item in some kind of semi-selected state. More applicable for acting on single images rather than groups.
• update focused image to the very last one in selected image list? It's a bit odd to increment the focus index after selecting a bunch of images and performing some action on them because it will increment to the 2nd selected image.