[DrJava] Line numbers finalized.

[Jim V. F. <big...@ow...>]

The basic idea is that we now use safer helper methods and massively
streamlined the code.  It runs slightly slower in two border cases,
jumping over large blocks of text and deleting blocks at the end of a
large file.

For most use, it is responsive, and it is always accurate.

We may work on some potential performance improvements; it is possible to
eliminate the second border condition, at least.

Enjoy!

Jim and Charlie