Resolve leak with two-layout text blocks

When we do a two-layout text block, we end up generating an initial
layout that gets found to not match the sizes and is discarded. That
layout wasn't getting properly disposed of.

This commit resolves #696.
Manual should show actual Shoes version
Exclude ::Swt::Graphics::GC from leak hunter

We don't create any instances of that class presently, and it generates
an undue amount of noise.

Can easily reinstate if we ever find we're making our own graphic
contexts.