Added JumpToLetter builtin

This commits 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)
![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.

There is also a strange issue that for skins the onclick event only gets triggered when the button control is double clicked by mouse. When using keyboard the onclick event is not triggered.  (this only applies to buttons with "jumpsmsX" and "jumpletterX" onclicks) Is this a known issue?

Cheers