Add examples
new file:   ch26/item04/DoubleBuffer.java
new file:   ch26/item05/TrackedImageLoad.java
new file:   ch26/item05/leonardo.jpg
new file:   ch26/item05/matisse.jpg
new file:   ch26/item05/picasso.jpg
new file:   ch26/item05/renoir.jpg
new file:   ch26/item05/seurat.jpg
new file:   ch26/item05/vincent.jpg
new file:   ch26/item07/HistoGrab.java
new file:   ch26/item07/vermeer.jpg