changed some function names

I'm going to change to another data structure (linked list) eventually,
so it would make sense for textbuf_current_line() to return a pointer to
the current line and not its number.
added get_line and current_line to TextBuffer

These functions return a pointer to the text of a specific line. These
will be useful once the lines are not stored in an array anymore.
added prev and next pointers to TextLine
lines are saved in a linked list
Enter works at end of line now
clear screen when deleting lines (fixes #2)