buffer->top_line could end up being NULL if we blindly TAILQ_PREV...

with the new implementation of redraw_window, we need to scroll a line more to "push" for a new page scroll.

redraw_window in ui.c already compute the curs_y by itself, so don't update it from here. Also, introduce forward_line: moves the cursor up/down n lines. It's amazing sometimes how a good interface can simplify a large chunk of code!

instead of using the line_off + vline_nth, cache the current top line.

before, the cursor went one line after the end and glitch everything.

buffer->current_line *IS NOT* the first line at the top of the window...

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.