Preparations for the Wordgraph version + minor fixes.

- Rearrange header file order.
- Validate that the words are not already split morphemes, to prevent an
  infinite loop. This is still not needed now, but is needed in the Wordgraph
  version.
- Use the suffix argument of add_alternative() for automatic INFIX_MARK
  prepending.
- int -> size_t for some variables.
- Fix Whitespace.
- Fix typos.
- Adapt the INFIX_MARK macro to its recent definition change.
Merge pull request #41 from ampli/backport

Preparations for the Wordgraph version + minor fixes.