Saw that layout really should be separated from the style struct, so went ahead and pulled the trigger...
A bunch of other refactors have also been done
* Lifted layout out of style, its not separate in UI_Box and in UI_State there is not a UI_LayoutCombo stack.
* UI_StyleTheme renamed to UI_StyleCombo
* UI_Theme has both UI_StyleCombo & UI_LayoutCombo
* Made files for the "project" related code
* ui_layout_compute moved to its own file, ui_layout now used for layout related data structures and interfacing
* Impovements to horizontal & vertical box impl
* UI_Box now keeps track of how many ancestors it has
