Added JumpToLetter builtin

This PR adds a new builtin called jumpletterX which would allow skinners to implement a neat alternative to the classic scrollbar. (still needs a little bit of python though to expose a list of all used sortletters to skinners, would be great to get that functionality into core, too)

SCREENSHOT:
![screenshot004](https://cloud.githubusercontent.com/assets/110931/4694533/41904394-57cd-11e4-9873-3dda5b453aca.jpg)

i implemented it in the same way as JumpSMS, if I should do it in a different way then please tell me. I only get into the C++ stuff if it helps me to get skin stuff done so I probably need some help from time to time to get the commits into shape :)

jumpletter[0-9] are not part of this PR, first want to know if it gets accepted at all.

Cheers
Please replace `65` with 'A' to make it more obvious what it means.