Polish image taking process

The photo button now shows the currently set image, if it's been taken
already.
Enable photography on all photo buttons; add labels
Add silhouette for person photo

This gives users a cue as to how to line themselves up with the camera
for the optimal picture.
Add step numbers to steps on top bar of voting booth
Add missing lib in CMakeLists.txt, remove blemish in
person_silhouette.png