unnecessary calls to dispose

[pixelzoom]

While investigating memory leak #87...

There are a bunch of places where dispose is called for subcomponent Nodes, and the calls appear to be unnecessary. This can be confusing when reading the code - why are some Nodes disposed and not others?

I'll put //TODO #199 comments in the code that mention this issue, and inspect them later.

[pixelzoom]

Removed 7 unnecessary dispose calls.
Documented 1necessarydispose` call.

Closing.