examples: Make possible to select the finger to enroll/verify

Move some common functions to an utilities file.

examples/meson.build

@@ -2,7 +2,7 @@
 examples = [ 'enroll', 'verify', 'manage-prints' ]
 foreach example: examples
     executable(example,
-               [example + '.c', 'storage.c'],
+               [example + '.c', 'storage.c', 'utilities.c'],
                dependencies: [libfprint_dep, glib_dep],
                include_directories: [
                  root_inc,