account: Always define all callbacks mentioned in .ui
Previously, webcam_icon_selected was only defined if gnome-initial-setup was built with Cheese support. However, it is always mentioned in the .ui file. This leads to the following warning at runtime:
Could not find signal handler 'webcam_icon_selected'. Did you compile with -rdynamic?
Define a dummy callback in the no-Cheese case. Warn if it is called. (The button should always be hidden if Cheese is not available, so it should never be called.)
Fixes #114 (closed).