added some utf-8 character functions
complete rewrite of insert_at_cursor()

It now writes the characters into the file buffer immediately after
reading them. This will make some things much easier in the future,
though it's less efficient than reading into a temporary buffer first.
backspace at beginning of line joins two lines
rewrote write_new_line()
u8_find_pos() works correctly now
insert_at_cursor() handles special characters better
move cursor with filebuf_move_cursor()
enter starts a new line