[Shotwell] [PATCH] Straightening tool addition

[Camilo Polymeris]

Hello,
attached is a patch that implements a "guide" for the straightening
tool, like offered by other applications: you can drag the pointer
along a line that should be vertical or horizontal & the the angle
will be adjusted accordingly (respecting the 15° limit).

This is mostly intended as an exercise for myself, while getting to
know the codebase & adapting to the coding style. Please have a look
at it and criticize if you have the time.

Regards,

Camilo