CamelStore: Handle CAMEL_STORE_FOLDER_CREATE flag directly.

camel_store_get_folder_sync() now handles this flag directly by first
calling get_folder_sync(), and on CAMEL_STORE_ERROR_NO_FOLDER it then
calls create_folder_sync() and retries get_folder_sync().

Subclasses will no longer receive a CAMEL_STORE_FOLDER_CREATE flag,
which should simplify things and yield more consistent behavior.

Solves https://bugzilla.gnome.org/724863, although from a different
angle than the way the problem description is written.