renamed FileBuffer to TextBuffer

FileLine is also changed to TextLine.

The reason for this change is simple: buffers are not connected to
files, so the name FileBuffer was a bit misleading. It's better to make
this change sooner than later.
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