HUD alignment and offset parameters

This adds the possibility to specify both alignment (relative to item size), and offset (in pixels, without regard to user's screen resolution). This would allow most (or all) kinds of layout, like specifying some stuff to be shown always on the bottom-left corner, or to use an "image" item as crosshair as in the following image:

https://dl.dropboxusercontent.com/u/100008207/screenshot_828120402.jpg