[GSoC15] Chained Text Boxes - Report Week 4

Hi there,
progress from this week revolved around:
- Refactoring overflowing code so to use an object having global
information on text chain;
- Implementing underflow handling during editing mode.

Underflow exploits overflow under the hood; current implementation is
basically a finite state machine that goes from underflow to overflow to
underflow again.

In some situations underflow handling behaves correctly but in others it
dies silently (for example after adding a character that keeps underflow
status).

Next step is looking into the issue above and deal with underflow while
drawing static text.

Cheers,
Matteo