Move Swt::TextBlock related classes to subdirectory

As noted in #629, I've been wanting to consolidate the SWT supporting TextBlock bits in a subdirectory for a while.

Apologies for the massive diff, but change in indentation forced it... try adding `?w=1` to the query string to see Github's whitespace aware diff.

Included so far:

* Moving the files around
* Simple renamings to remove unnecessary `TextBlock` prefixes

There's one more change I'd like to include that I would love input on. The names for `FittedTextLayout` and `FittedTextLayoutCollection` have been grating on me for a while. Those classes aren't just a layout wrapper anymore--I consider them to be representative of a piece of the text block (incidentally backed by a single Swt::TextLayout, but that's implementation, not intent).

I'd like to choose more meaningful name. Here are some possibilities, and I'm keen to hear your thoughts on it since a big motivator is making the purpose clearer to people who aren't me :grinning: 

* `Shoes::Swt::TextBlock::Segment`
* `Shoes::Swt::TextBlock::Section`
* `Shoes::Swt::TextBlock::TextSegment`
* `Shoes::Swt::TextBlock::PartialBlock`
* Your best idea???