Remove incorrect sizing methods for text block

The original text block implementation had a bad design where the sizes were
asked immediately from the SWT version before anything had been fitted. With
text flowing, that can't give the right answer any longer.

This commit removes that cruft, and at least one nil check along the way that
became superflous with that code gone.

Things are generally working, although the text block DSL now lacks the
proper dimensions which will get resolved in a later commit.
Text fitting sets back calculated with to dsl
Text returns calculated width if width isn't set
Abide by explicit width when fitting

The DSL for TextBlock now calculates the necessary width, taking into account
if we've set the desired value explicitly, falling back to the parent if not.

Fixes #514.