for the time being this seems good enough. In the future we may want to expand a tab into N spaces tho.

- commands now take a struct window* instead of a raw tab - the struct ui_state is gone - using a window for the minibuffer too helps avoid code duplication - it will let us having more than one window on a tab! :DD It doesn't seem to have broken anything...

the code relative to the cursors movements now respects the width of the characters (zero, one or two cells).