grafx - Extensible Java Graphics Package Code

Adding some anal checks & proofreading code in CanvasSplitter. Adjusting one
'while' loop - using 'do ... while' in its place because that is more correct.
Removing TODO comment because the logic is correct, or it seems to be so, after
close examination. The original code was actually fine.

Modifying the formatting in Javadoc comments.

Adding some comments to CanvasSplitter.

In NightmareLevelTest, removing private inner classes 'SidebarUpdatable' and
'NextBlockUpdatable', replacing with 'SubUpdatable'.

In LevelTest, removing classes 'SidebarUpdatable' and 'NextBlockUpdatable',
replaced with class 'SubUpdatable' defined in CanvasSpitter, the parent class.

In ComputerPlayerTest, removing private inner classes 'SidebarUpdatable' and
'NextBlockUpdatable', and using class 'SubUpdatable' instead.

Removing private inner classes 'SidebarUpdatable' and 'NextBlockUpdatable' from
levelPlayback. Using SubUpdatable (defined in CanvasSplitter) instead.

Removing external class SubUpdatable as promised.

Committing external class SubUpdatable. The intention is to immediately remove
this class thereafter. Saving it in case I need to resurrect it later.

Adding inner protected static class SubUpdatable to CanvasSplitter so that code
would not have to be repeated. Not using this yet. Other changes are minor;
changing some local variables to final, changing signatures of some methods
from 'public' to 'protected'.