Camel has many struct members and function parameters named 'flags'.
Using a plain integer type for these can lead to confusion about which
flag definitions are expected.

This collects flag values into enumeration types, and changes the
struct member or function parameter type from a plain integer type
to the proper enumeration type.

TODO: Collect enum types into a camel-enums.h file and let GObject
      generate GTypes for each of them so they can be used properly
      in GObject properties, GtkTreeModel columns, etc.