Change scaling mode option

This patch will expand functionality of "letterbox" button.
Now it is a Combobox. You can choose "Letterbox", or "Stretch", or new option - "Expand"
It will expand base resolution so it will fill all the screen. Effectively removing black bars from sides or top/bottom, preserving object sizes and aspect ratio.
That, for example, could be useful  for android port.
It would remove side black bars when using original base resolution (SCALE_SCREEN_DIV_3 is still too small for phone and phablets).
![nonsquare](https://cloud.githubusercontent.com/assets/858810/3489161/d8ca338e-0519-11e4-98ee-500d43b77709.gif)
![square](https://cloud.githubusercontent.com/assets/858810/3489162/d8ca92c0-0519-11e4-9f12-9fc470956658.gif)