kilo: refactoring access to g_editor_state

I've been continuing working through the steps to implement kilo, but I've been increasingly unhappy with the accesses to the global g_editor_state variable all over the code; it's ugly and makes too much of the code dependent on the precise declaration of that struct.

So I decided to refactor so that all accesses to the editor state are through aptly named functions. I'm a lot happier with the result. As an example, here's the current implementation of editor_draw_rows:

void editor_draw_rows(term_buffer* tb)
{
    for (int y = 0; y < get_screen_height(); y++)
    {
        if (y < get_file_lines())
        {
            int len = get_line_size();
            if (len > get_screen_width())
              len = get_screen_width();
            tb_append(tb, get_line_chars(), len);
        }
        else if (y != get_screen_height()/3)
            tb_append_str(tb, get_tilde_str());
        else if (get_file_lines() == 0)
            append_welcome_message(tb);
        
        tb_append_str(tb, get_clear_row_str());
        if (y + 1 < get_screen_height())
            tb_append_str(tb, get_rn_str());
    }
}