UI: Text wrapping is not working correctly, very slow

[vxlcoder]

The layout algorithm does not work well when text elements are interrupted with other elements. Also, the current implementation is awfully slow!

rewrite the layout system using better data structures.

notes: the layout system is called twice. first time is to compute sizes and break inlines into lines. the second time sets the position of the element (center/right alignment, table cells, etc). instead, layout_size should be called from UI_Document, then a layout_position should be called from UI_Document.