now, instead of prefix and line style, I'm adding the trailing style too. This changed the configuration syntax too.

currently unused, will help later when adding colour support

this detangles definitely the commands from the ui. Various variables and struct from ui.c needed to be exported, aside from that, no functional changes.

don't redraw the page if no scrolling happened. There's still need to ``force'' the redraw sometimes, like after a resize or after toggling olivetti-mode, so add a buffer->force_redraw flag for it.

i.e. don't hardcode text attributes

and: * add some flags * document them * move the customizable stuff to defaults.c