Commit Briefs
fix end-of-buffer glitch
before, the cursor went one line after the end and glitch everything.
fix cmd_scroll_line_up
buffer->current_line *IS NOT* the first line at the top of the window...
move all cmd_* functions in their own file
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.